Selecting One of Several Mating Types through Gene Segment Joining and Deletion in Tetrahymena thermophila
Figure 7
Model proposing that homologous recombination assembles a single mating type gene pair during somatic differentiation.
In this model, intramolecular recombination events are initiated at both ends of the germline array; subsequent resolution results in removal of intervening gene pairs by looping out and joining of a gene pair to the full length TM exons at the ends of the array. Any number of gene pairs could be excised in a single recombination event; since the chromosomal product regenerates the recombination substrate, recombination steps can be reiterated until a single, complete gene pair remains, at which point the process has to stop. Sequestering or disabling the ability of side products to recombine again would minimize unproductive reversal of the process. Recombination events need not always involve a full length TM exon; two internal tm exons could also be involved at intermediate steps. The recombination process is labeled “homologous recombination” for simplicity, but identical results could be obtained by highly precise non-homologous end-joining. Side products containing a discrete number of gene pairs, shown here as circular, could also be linear depending on the details of the recombination and repair mechanism. A related DNA rearrangement model of T. thermophila mating type determination, also involving recombination and alternative deletion in a tandem array of germline mating type genes was proposed previously [14]. The key conceptual difference is that in the original model a unique segment was somatically attached at one end of an individual mating type gene, instead of attaching unique segments at both ends of a mating type gene pair, as reported here.