Presynapsis and synapsis of DNA promoted by the STP alpha and single-stranded DNA-binding proteins from Saccharomyces cerevisiae.

We previously purified an activity from meiotic cell extracts of Saccharomyces cerevisiae that promotes the transfer of a strand from a duplex linear DNA molecule to complementary circular single-stranded DNA, naming it Strand Transfer Protein alpha (STP alpha) (Sugino, A., Nitiss, J., and Resnick, M. A. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 3683-3687). This activity requires no nucleotide cofactor but is stimulated more than 10-fold by the addition of yeast single-stranded DNA-binding proteins (ySSBs). In this paper, we describe the aggregation and strand transfer of double-stranded and single-stranded DNA promoted by STP alpha and ySSB. There is a good correlation between the aggregation induced by various DNA-binding proteins (ySSBs, DBPs and histone proteins) and the stimulation of STP alpha-mediated DNA strand transfer. This implies that the stimulation by ySSBs and other binding proteins is probably due to the condensation of single-stranded and double-stranded DNA substrates into coaggregates. Within these coaggregates there is a higher probability of pairing between homologous double-stranded and single-stranded DNA, favoring the initiation of strand transfer. The aggregation reaction is rapid and precedes any reactions related to DNA strand transfer. We propose that condensation into coaggregates is a presynaptic step in DNA strand transfer promoted by STP alpha and that pairing between homologous double- and single-stranded DNA (synapsis) occurs in these coaggregates. Synapsis promoted by STP alpha and ySSBs also occurs between covalently closed double-stranded DNA and single-stranded linear DNA as well as linear double-stranded and linear single-stranded DNAs in the absence of any nucleotide cofactors.


Presynapsis and Synapsis of DNA Promoted by the STPa and Singlestranded DNA-binding Proteins from Saccharomyces cereuisiae"
(Received for publication, January 13, 1989)

Research Triangle k'ark, North Carolina 27709
We previously purified an activity from meiotic cell extracts of Saccharomyces cerevisiae that promotes the transfer of a strand from a duplex linear DNA molecule to complementary circular single-stranded DNA, naming it Strand Transfer Protein a (STPa) (Sugino, A., Nitiss, J., and Resnick, M. A. (1988) Proc. Natl. Acad. Sci. U. S. A. 85,3683-3687). This activity requires no nucleotide cofactor but is stimulated more than 10-fold by the addition of yeast single-stranded DNA-binding proteins (ySSBs). In this paper, we describe the aggregation and strand transfer of doublestranded and single-stranded DNA promoted by STPa and ySSB. There is a good correlation between the aggregation induced by various DNA-binding proteins (ySSBs, DBPs and histone proteins) and the stimulation of STPa-mediated DNA strand transfer. This implies that the stimulation by ySSBs and other binding proteins is probably due to the condensation of singlestranded and double-stranded DNA substrates into coaggregates. Within these coaggregates there is a higher probability of pairing between homologous double-stranded and single-stranded DNA, favoring the initiation of strand transfer. The aggregation reaction is rapid and precedes any reactions related to DNA strand transfer. We propose that condensation into coaggregates is a presynaptic step in DNA strand transfer promoted by STPa and that pairing between homologous double-and single-stranded DNA (synapsis) occurs in these coaggregates.
Synapsis promoted by STPa and ySSBs also occurs between covalently closed double-stranded DNA and single-stranded linear DNA as well as linear doublestranded and linear single-stranded DNAs in the absence of any nucleotide cofactors.
DNA strand exchange proteins are an important category of proteins that have emerged in the last decade (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11). They have been studied in several organisms and mammalian cells and are expected to mediate homologous DNA recombination, but this has been shown genetically only for prokaryotes and Ustilago maydis (1). With a large base of genetic data on recombination processes, Saccharomyces cerevisiae is well suited for an in-depth analysis of DNA recombination at the molecular level. Several types of genetic recombination have been identified in yeast (12) involving many genes, some of which may code for activities that catalyze the reactions * The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "aduertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
$ involved in recombination (13). Recently, we have identified and purified to homogeneity (9) an activity from yeast meiotic cells that promotes DNA pairing and strand transfer between a linear double strand and a complementary, circular single strand and have named this protein yeast strand transfer protein a (STPa).' This activity does not require any nucleotide cofactor but is greatly stimulated by the addition of yeast single-stranded DNAbinding proteins. Kolodner et al. (8) have also identified, and purified from mitotically growing yeast cells, a protein that promotes an ATP-independent strand exchange reaction. The relationship between the activity from meiotic cells and that from mitotic cells is not known at this point.
Since DNA strand exchange reactions previously described in other systems require ATP (1, 2, 4-6), a major question about the ATP-independent DNA strand exchange reaction promoted by STPa and ySSB is whether it requires energy and, if so, from what source? T o gain some insights into this ATP-independent reaction, we present here an investigation into the stimulatory effect of ySSBs during the DNA strand exchange reaction promoted by STPa. We find that in addition to ySSBs, any agent that generates coaggregates between ss-and dsDNA also stimulates DNA strand transfer promoted by STPa. We will refer to the product formed from the concurrent and coincident aggregation of single-stranded and double-stranded DNA as coaggregates. Examples of such agents are other yeast DNA-binding proteins, histone proteins, and spermidine. SSBs that cannot aggregate DNA also do not stimulate STPa activity. These results strongly suggest that the formation of coaggregates is a presynaptic step for ATP-independent DNA strand transfer in yeast. While S T P a is not required for presynapsis, it is required for pairing between homologous double-and single-stranded DNA (synapsis). STPa also promotes ATP-independent strand transfer between a covalently closed dsDNA and linear ssDNA in the presence of ySSB. Based on these observations, we will discuss a model for the mechanism of DNA strand transfer promoted by STPa and ySSB. arose, gel filtration, and Mono S.' These proteins were more than 95% pure as judged by analysis of Coomassie Blue staining of SDSpolyacrylamide gels and did not contain any detectable DNase (either endo-and exonuclease) or topoisomerase activities under the conditions used for measuring the DNA strand transfer reaction. Yeast DNA-binding proteins DBP I, 11, and 111 (16,34) were obtained from Drs. T. Goto and J. C. Wang, Harvard University. Calf thymus histone H1 and other histone proteins were purchased from Boehringer Mannheim, respectively. Yeast histone proteins were purified as described before (17). Restriction endonucleases, T4 polynucleotide kinase, bacterial alkaline phosphatase, 4x174 viral single-stranded DNA, its RF-I and RF-I1 DNA were purchased from Bethesda Research Laboratories. Restriction endonuclease-linearized RF DNA (RF-111) of both 4x174 and M13mp18 were the same as previously described (9). @X174 linear viral single-stranded DNA was prepared as follows: 250 pg of @X174 viral single-stranded circular DNA was annealed in a reaction mixture (1.25 ml) containing 20 mM Tris-HC1, pH 7.9, 10 mM MgCI,, 100 mM NaCI, 2 mM dithiothreitol at 65 "C for 60 min with 20 pg of an oligonucleotide containing the XhoI recognition site complementary to nucleotides 154 and 175 of @X174 viral DNA (18). The reaction mixture was cooled slowly to room temperature, 500 units of XhoI was added, and the incubation was continued for another 60 min at 37 'C. After the addition of EDTA to 10 mM, the product was treated with phenol, precipitated with 70% ethanol, and resuspended in 10 mM Tris-HC1, pH 7.5, 1 mM EDTA. The 22-mer oligonucleotide was synthesized with a Vega Coder 300 DNA synthesizer.
32P-Labeled 4x174 viral DNA was prepared as follows. Escherichia coli C cells were grown at 37 "C in 500 ml of low phosphate media (19) to 2.5 X IO* cells/ml at 37 "C. Five min after the addition of 5 mCi of ["Plphosphate, 4X174am3 phage were added at a multiplicity of infection of 2.5. After 3.5-h incubation at 37 "C, the cells were pelleted in a GSA rotor at 10,000 rpm for 10 min, resuspended in 10 ml of 0.1 M sodium borate, and lysed by the addition of 10 mg of lysozyme and 1 ml of 4% EDTA and incubation at room temperature for 90 min. The lysate was sonicated with 3-s pulses until the viscosity was reduced. CsCl was added to 0.6 g/ml, and the sample was centrifuged in a Beckman VTi50 rotor for 18 h at 40,000 rpm. The opalescent band at the middle of the tube was collected and dialyzed against 0.05 M sodium borate. The phage were rebanded on a step gradient of CsCl in a SW41 rotor at 40,000 rpm for 12 h, collected, and dialyzed against 0.05 M sodium borate. The DNA was extracted from the phage by phenol. The specific activity of "P-labeled DNA was about 1.5 X lo5 cpm/pg. The 32P-labeled viral DNA was linearized in the same way as described above. 3H-Labeled 6x174 viral ssDNA was prepared as published (20). DNA Strand Transfer Reaction-The reaction mixture (0.02 ml) contained 35 mM Tris-HCI, pH 7.9, 10 mM MgCl,, 2 mM dithiothreitol, 0.1 mg/ml bovine serum albumin, 0.2 pg of @X174 viral ssDNA, 0.4 pg of 4x174 RF-111 DNA (9), 2.4 pg of the 26-kDa ySSB and 20-40 ng of STPa (Fraction VI) (9). The reactions were carried out at 30 "C for the times indicated. In order to analyze the reaction products by agarose gel electrophoresis, the reactions were terminated by the addition of 1% NaDodSOI, 10 mM EDTA, and 5% glycerol, and the samples were directly applied to a 1% agarose gel (9). Occasionally, the samples were further treated with 0.1 mg/ml Proteinase K followed by phenol extraction. However, the migration of the products upon agarose gel electrophoresis was not affected by this treatment.
DNA Aggregation-The extent of DNA aggregation caused by STPa and DNA-binding proteins was measured in the same reaction mixture as the DNA strand transfer reaction containing "P-labeled 4x174 RF-I11 DNA (9) and 3H-labeled 4x174 viral single-stranded circular DNA according to published procedures (21). Briefly, after the incubation, the reaction mixtures were centrifuged at room temperature for 1 min in an Eppendorf minifuge, and the radioactivity in the supernatant was measured by a liquid scintillation spectrometer. Electron Microscopy-After reacting, the DNA samples were prepared for electron microscopy by treatment with 1% NaDodS04, 10 mM EDTA, 0.1 mg/ml Proteinase K for 20 min at room temperature and extraction with an equal volume of phenol. The samples were then purified through a Bio-Gel A5m column as previously described (22,23). Formamide spreading and platinium shadowing of the DNA molecules was the same as published by Chow and Broker (24). Molecules were viewed with a Jeol 100s electron microscope and length measurements were made as published (22). ' R. K. Hamatake and A. Sugino, unpublished results.
Other Methods-Other methods in this study were the same as described previously (9).

DNA Aggregation Is a Crucial
Step for DNA Strand Transfer Catalyzed by STPa-As is observed for the RecA system (21,25), DNA aggregation may be required for the ATPindependent DNA strand transfer promoted by STPa. We therefore determined DNA aggregation and STPa-promoted strand transfer in response to the 26-kDa ySSB. Although the 20-kDa ySSB was used previously for stimulation of STPa activity (9), we use the 26-kDa ySSB in these studies because it is the native form of the 20-kDa ySSB.* We have not detected any biochemical difference between them so far. DNA aggregation was measured as the fraction of DNA that pellets in a 1-min centrifugation (21). DNA strand transfer activity was measured at the same time using the agarose gel electrophoresis assay (9). As the concentration of 26-kDa ySSB increased, there was a sharp transition from no aggregation to complete aggregation in the presence of both dsand ssDNA; the transition midpoint was about 120 pg/ml ( Fig. 1). DNA strand transfer activity showed a similar dependence on the concentration of 26-kDa ySSB. There was a correlation between the stimulation of activity and the aggregation curve up to 120 pg/ml, with higher concentrations of 26-kDa ySSB inhibiting the DNA strand transfer reaction ( Fig. 1). In the presence of ssDNA alone, at the same concentration that was used in the strand transfer assay, about 30 pg/ml ySSB aggregated 50% of the ssDNA, while approximately 90 pg/ml of ySSB was able to aggregate 50% of dsDNA ( Fig. 1). Aggregation of ds-and ssDNA therefore requires the total amount of ySSB necessary to aggregate both 8s-and dsDNA alone. Unlike strand transfer, aggregation of both dsand ssDNA did not require homologous DNA combinations:  Fig. 1, the addition of STPa had no effect on DNA aggregation (data not shown). We conclude that DNA aggregation is primarily a property of the 26-kDa ySSB and not of STPa.
A requirement for aggregation of both ds-and ssDNA as a prerequisite for the strand transfer reaction is suggested by the experiments shown in Figs. 2 and 3. Aggregation of dsand ssDNA by ySSB and STPa showed the same salt sensitivity as DNA strand transfer (Fig. 2) in a manner similar to that seen for the RecA protein (26). The DNA aggregation reaction in the presence of 26-kDa ySSB is rapid with the reaction complete within 5 min at 30 "C and precedes the DNA strand-transfer reaction (Fig. 3). Therefore, these data are consistent with the idea that formation of coaggregates is a crucial step for DNA strand transfer catalyzed by STPa.  Table I. If DNA aggregation is due to counterion condensation (29), then acidic proteins are not expected to be good DNA aggregating agents. The isoelectric point for both the E. coli SSB and T4 gene 32 proteins is acidic (27), and we did not observe the formation of DNA aggregates by these proteins. For these two proteins, stimulation of the DNA    strand transfer reaction catalyzed by STPa was less than 2fold.
DNA-binding protein I (DBP I) of yeast is required for the catenation reaction catalyzed by yeast Topoisomerase I1 (16).
This implies that DBP I also generates DNA aggregates, since DNA aggregation is required for the catenation reactions catalyzed by all known DNA topoisomerases (28). We expected that DBP I would also stimulate the strand transfer reaction promoted by STPa. Surprisingly, yeast DBP I showed very poor aggregate formation and poor stimulation of DNA strand transfer (Fig. 40). Neither DNA aggregation nor stimulation of the DNA strand-transfer reaction could be observed in the presence of DBP I11 (Fig. 4F). On the other hand, DBP I1 was a good DNA aggregant, and the concentration giving roughly 50% aggregation of both ds-and ssDNA maximally stimulated the DNA strand transfer reaction (Fig.  4E). Among the yeast SSBs, the 14-, 26-(a native form of 20-kDa), and 35-kDa ySSBs were the best for both DNA aggregate formation and DNA strand transfer although the optimal concentration of each ySSB was different (Figs. 1 and 4). The remaining ySSBs neither stimulated DNA strand transfer nor formed DNA aggregates. As shown in Fig. 5, histone H1 from calf thymus is a good aggregant and also stimulates DNA strand transfer catalyzed by STPa. However, some differences were observed between ySSB and calf histone H1 protein: the same concentration of histone H1 is required for the aggregation of either ss-or dsDNA (Fig. 5), the maximum stimulation of pairing is 2-5fold lower, and the transition midpoint from no aggregation to complete aggregation is not as sharp as for the 26-kDa ySSB. Other calf thymus histone proteins (H2A, H2B, H3, and H4) and yeast histone proteins (a mixture of all histone proteins) also aggregated DNA and stimulated DNA strand transfer promoted by STPa (Table I).
The proteins used in this study are similar in their affinity for binding to DNA but have different effects on the DNA once they are bound. In all cases, those proteins that are able to aggregate DNA stimulate STPa-promoted strand transfer. Conversely, those proteins that are incapable of aggregating   Spermidine Can Substitute for ySSB to Stimulate DNA Strand Transfer Promoted by STPa-Spermidine is known to aggregate DNA and to stimulate the DNA catenation reaction catalyzed by DNA topoisomerases (29). If DNA strand transfer promoted by STPa requires a similar sort of DNA aggregate, then spermidine should also stimulate the strand transfer reaction. As shown in Fig. 6, spermidine not only generates DNA aggregates, but also stimulates the strand transfer reaction catalyzed by STPa. Although the maximum stimulation was 4-fold lower than that seen with the 26-kDa ySSB, spermidine was similar to the other aggregating agents in stimulating maximum strand transfer activity at a concentration that causes approximately 50% DNA aggregation. Both DNA aggregation and DNA strand transfer in the presence of spermidine were very salt sensitive, with 50% inhibition by 50 mM KC1 (data not shown). This is similar to the observations of DNA aggregation and catenation of relaxed DNA by E. coli DNA gyrase (29).
DNA Synapsis by STPa-After establishing a presynaptic complex (DNA aggregation), synapsis formation (pairing of homologous DNA) could be the next step during the DNA strand exchange reaction promoted by STPo. We have analyzed the synapsis reaction by measuring the formation of stable "D-loop" structures using 4x174 RF-I and its linear viral DNA by both agarose gel electrophoresis (22,30 electron microscopy. As shown in Fig. 7A, new DNA bands migrating much more slowly than either nicked or covalently closed, relaxed RF DNA were produced by STPa in the prescence of ySSBs. Although these were evident on an ethidium bromide-stained agarose gel, they were much clearer on the autoradiograph of the dried gel (Fig. 7B). The formation of these bands required Mg", homologous DNA, and stoichiometric amounts of ySSB, but no nucleotide cofactor, in a manner identical to the conditions for the strand transfer reaction between linear dsDNA and circular ssDNA (9). To verify that these bands represent DNA synapsis products (e.g. D-loop structures), the reaction products were examined by electron microscopy. Fig. 8 shows some representative DNA molecules seen in the products. As shown in Table 11, about 10% of dsDNA was an initial synapsis molecule (joint molecule) which has a single strand tail without an obvious D-loop structure and about 5% was a D-loop structure with varying sizes of paired regions and one or two tails of ssDNA (Fig. 8,  B-E). Most of the remaining DNAs were the structures expected from the starting materials (supercoiled ds-and linear ssDNAs) (Fig. 8 A ) or large aggregates. If the DNA was not treated with Proteinase K and/or NaDodSO, before electron microscopy, only large aggregates were seen, as expected from the action of the 20-or 26-kDa ySSB (data not shown).

Strand Transfer between Linear ds-and Linear SSDNAS-
As shown in Fig. 9, left, STPa and ySSB promote strand transfer between 4x174 RF-I11 DNA and 4x174 linearized viral ssDNA. The new DNA bands can be seen easily on an agarose gel. Autoradiography of the dried gel confirms that these new bands are in fact joint molecules between RF-I11 and linear ssDNA (Fig. 9, right). The joint molecule formed from RF-I11 and linear ssDNA migrates similarly to the joint molecule formed from RF-I11 and circular ssDNA because of their equivalent mass. Conversion of the RF-I11 DNA to joint molecules was almost complete. Like the previously described reactions, this strand transfer required homologous DNAs, M P , and ySSB (data not shown).

DISCUSSION
DNA strand transfer is believed to be one of the steps involved in homologous DNA recombination (1). Thus, proteins which promote DNA strand transfer in vitro may be important for homologous recombination in both prokaryotic and eucaryotic systems. In some cases, this has been proven genetically (1). In these cases, the proteins all require a nucleotide cofactor for the reaction and are DNA-dependent NTPases. Recently, however, strand transfer proteins have been isolated from human (3), yeast (8,9), and Drosophila melanogaster cells (10, 11) which differ from the previously described strand transfer proteins in not requiring any nucleotide cofactors. STPa from s. cereuisiae meiotic cells contained no detectable endo-or exonuclease or ATPase activities and promoted extensive strand transfer in the presence of ySSB (9). We have suggested that strand transfer does not require energy from ATP hydrolysis and that the mechanism is different from that of the previously described reactions.   (left-hand figure), followed by autoradi- ography (right-hand figure) as in Fig. 7 The possibility that STPa has an unidentified high energy source has not yet been ruled out. This is, however, unlikely because each STPa molecule promotes the transfer of more than 2000 base pairs (9) and any inherent energy should be quickly consumed at the beginning of the reaction.
In order to gain a further understanding of this ATPindependent strand transfer reaction, we have studied the STPa-promoted reaction in more detail. We have found that yeast DNA-binding proteins or spermidine play a crucial role for strand transfer promoted by STPa. Among the ySSBs and DBPs tested in this paper, the 14-, 20-, 26-, and 35-kDa ySSBs and DBP I1 stimulated the strand transfer reaction significantly (more than 20-fold). These proteins also promoted the formation of DNA aggregates. Histone H1 and spermidine, which are known to be good DNA aggregants (29), also stimulated DNA strand transfer to almost the same extent as ySSBs. The correlation between aggregation of both ds-and ssDNA and stimulation of DNA strand transfer implies that coaggregates of ss-and dsDNA are readily accessible to STPa and that within these coaggregates the search for homology is facilitated by the high local concentrations of DNA. Optimal concentrations of various DNA-binding proteins and spermidine for the strand transfer reaction catalyzed by STPa is approximately the amount that generates 50% substrate DNA aggregation.
Concentrations resulting in higher than 50% aggregation inhibited DNA strand transfer. This is similar to the observations on the catenation reaction catalyzed by DNA topoisomerases (29). What significance does the aggregation of DNA have to homologous pairing in uiuo? In all of the in uitro DNA pairing reactions reported thus far (9 and in this report), the substrate DNA concentrations are on the order of micromolar, far less than the millimolar DNA concentrations expected to be present in the yeast nucleus. Therefore, one interpretation of why DNA aggregation facilitates DNA strand transfer promoted by STPa in uitro is that a high DNA concentration is necessary to support efficient homologous pairing and that aggregation promotes these pairing conditions. Since the effective DNA concentration in uiuo is already very high, this interpretation implies that DNA aggregation and agents that induce aggregation may not have any specific relevance to in uiuo pairing. Alternatively, the formation of an aggregated complex of DNA and proteins, such as nucleosome structure, could be integral to the location of homologous sequences in uiuo and may help and Synapsis 13341 account for the remarkable efficiency of homologous recombination in yeast. Based on the data so far (9 and this study), a model for the mechanism of the DNA strand transfer reaction promoted by STPa and ySSB is presented in Fig. 10. The first stage of the reaction is the binding of ySSB to DNA and subsequent presynaptic complex formation (DNA aggregation). Here, we predict that 20-or 26-kDa ySSB binds to ssDNA first, due to preferential binding to ssDNA over dsDNA (13). This stage does not require STPa or homologous DNAs. The second stage involves searching for homology and formation of synaptic complexes (D-loop structures) which absolutely require homologous DNAs and STPa. It is not yet known whether paranemic joints are made before the formation of plectonemic joints (stable D-loop structures) (1). Technically it has been impossible to detect paranemic joints by filter binding experiments and/or electron microscopy, since ySSB alone aggregates DNAs so efficiently at the early stages of the reaction. The last stage presented here is branch migration to form a final product, an RF 11-like molecule and displaced linear ssDNA. More complicated products are commonly observed in the reaction suggesting that the displaced ssDNA might be a preferred substrate for further invasion of dsDNA, leading to a network of partial strand transfer products (8). This inefficient completion of strand transfer may be related to the absence of a nucleotide cofactor requirement for the reaction or to the lack of some other factor(s) required for complete DNA strand transfer. The T4 uusX protein, like STPa, also produces complex DNA strand transfer reaction products under certain conditions (32, 33). The T4 uusX protein produces only a small amount of RF I1 DNA and, in the presence of T4 gene 32 protein, the displaced linear ssDNA further invades dsDNA to form complicated networklike structures during strand transfer (32, 33). Unlike STPa, however, T4 uvsX protein efficiently generates a typical RF I1 DNA and a free displaced linear ssDNA in the presence of E. coli SSB instead of gene 32 protein (32).
By comparing the reaction of STPa and ySSB with that promoted by E. coli RecA protein (and E. coli SSB), some major differences are evident besides the lack of a nucleotide cofactor requirement. One of these is the role of ySSB during strand transfer. In the case of the reactions promoted by RecA, the current model has RecA molecules binding first to ssDNA, forming a special conformation of the DNA. Subsequently, E. coli SSB displaces RecA protein coated on ssDNA before the reaction can proceed. Thus, for RecA it is very important when and in which order each of the proteins are added (1). The DNA strand transfer catalyzed by STPa and ySSB does not require any specific order of addition. Since a few molecules of STPa promote extensive DNA strand transfer, it is likely that ssDNA first interacts with ySSB and subsequently STPa displaces ySSB molecules on ssDNA during the reaction. ySSB itself generates DNA aggregation which could be a crucial step for the initiation of the strand transfer reaction, while RecA protein alone generates DNA aggregation. We speculate that ySSB might share a part of the roles played by E. coli RecA protein during DNA strand transfer. Another difference is that that the levels of RecA protein increase during recombination while the amount of STPa protein remains low, and constant, during meiosis although its specific activity increases (data not shown). The mechanisms for increasing the levels of strand transfer activity when recombination is induced appear to differ in yeast and E. coli.
Our current work involves the comparison of the mitotic and meiotic STP activities, both biochemically and genetically. The preliminary evidence indicates that the mitotic activity acts in a very similar manner to the meiotic activity but with a lower efficiency. This correlates well with genetic observations of recombination frequencies during mitotic growth uersus meiosis (13).