Ironing out the kinks: splicing and translation in bacteria
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2021 USA
This extract was created in the absence of an abstract.
In eukaryotic cells, RNA processing is physically separate from protein synthesis. As nuclear export of unspliced RNA is restricted, only mature messages are normally exposed to the translational machinery. In bacteria, however, splicing must be coordinated with the translation of nascent transcripts. These two processes make very different demands on the RNA substrate: Splicing of autocatalytic introns requires that the 5′ and 3′ splice sites be brought together as part of an elaborate tertiary structure, whereas translation requires that the mRNA be relatively free of secondary structure. Nonetheless, introns have been found in highly expressed genes in eubacteria, bacteriophages, mitochondria, and chloroplasts (for review, see Burke 1988). Clearly, there must be some means of balancing splicing of bacterial introns with cotranscriptional translation. In this issue, Semrad and Schroeder (1998) provide the surprising answer that splicing of a group I intron from phage T4 is facilitated by translation of the upstream open reading frame (ORF). This enhancement of splicing is achieved by modulating the long-range conformation of the pre-mRNA. Their results provide useful analogies for the coupling of eukaryotic pre-mRNA splicing with transcription.
Bacterial mRNAs exclusively contain group I or group II introns, and the three group I introns that are present in phage T4 are all able to self-splice in vitro (for review, see Belfort 1990). The introns are found in genes encoding thymidylate synthase (td), ribonucleotide reductase (nrdB) (Belfort 1990), and anaerobic ribonucleotide reductase (sunY, or nrdD) (Young et al. 1994). In addition to sequences that provide the necessary functions of self-splicing, td and sunY intrans contain internal ORFs that encode double-stranded DNA endonucleases (Belfort 1989). The endonucleases trigger homing, or site-specific movement of the intron sequences to intronless alleles.
Self-splicing requires that the intron RNA fold into a unique secondary and tertiary structure (Cech and Herschlag 1996 …
