Messenger RNA stability in prokaryotes has been shown to be an important mechanism by which gene expression is controlled at the post-transcriptional level. Over the last few years data have accumulated indicating that in E.coli variation in stability between mRNA species is determined by the susceptibility of a given transcript for endo- or exoribonucleases. Among the endoribonucleases that control gene expression at the mRNA level, RNase III plays an important role. Unlike RNase E, however, RNase III does not affect bulk mRNA stability. RNase III acts as a dimer to cleave double-stranded RNA and it was shown recently that RNase III is membrane associated. RNase III is involved in processing the 30S rRNA precursor.
The expression of several cellular, phage and plasmid genes is known to be affected by RNase III. An example of a chromosomal gene that is negatively regulated post-transcriptionally by RNase III is the rnc gene encoding RNase III itself RNase III cleavage sites have also been demonstrated for the rplL-rpoB, dicA-dicF-dicB and the metY-nusA intercistronic regions. Finally, it was shown by Robert-Le Meur and Portier (1992) that the expression of the pnp gene, encoding polynucleotide phosporylase, a 3´ to 5´ exoribonuclease, is autoregulated through an RNase III dependent mechanism. Among the phage genes that are regulated by RNase III is the bacteriophage l N gene which is activated by RNase III, the l int gene which is inhibited by RNase; and gene 1.2 of bacteriophage T7. RNase III might also play a role in antisense-control of gene expression: The sense-antisense RNA duplex in the IS10 RNA-OUT/RNA-IN system which controls expression of the IS10 transposase is cleaved by RNase III. However, even though it was shown that the cleavage destabilizes RNA-IN it is not required for antisense control in vivo. Despite these broad range of activities that are associated with RNase III, it was demonstrated very recently by Babitzke et al. (1993) that RNase III is not absolutely required for cell viability, general mRNA degradation, or rRNA processing.
Gene 19 of the conjugative resistance plasmid R1, which is a member of the plasmid incompatibility group IncFII, has also been designated as "gene X" (Thompson et al., 1984) and "ORF169" (Loh et al., 1989). It is located immediately adjacent to the origin of transfer which is believed to be cleaved by a highly ordered nucleoprotein complex in a strand- and site-specific fashion. As a part of the so called "leading region", gene 19 is the first gene to be transmitted during the process of conjugal DNA transfer into the recipient cell. The function of gene 19 is presently unknown, but results obtained in our laboratory show that it acts as an important element in the conjugation process (gene 19 function).
We could show that expression of gene 19 is posttranscriptionally controlled by RNase III. We demonstrated that the cleavage occurs in the coding part of the gene 19 transcript whereas in all other known examples of mRNAs that are processed by RNase III cleavage takes place in noncoding regions. Our findings suggest that by affecting gene 19 expression RNase III also may influence conjugal DNA transfer and underscore the very broad range of effects mediated by RNase III.
G. Koraimann, Ch. Schroller, H. Graus, D. Angerer, K. Teferle und G. Högenauer (1993) Expression of gene 19 of the conjugative plasmid R1 is controlled by RNaseIII. Mol. Microbiol. 9(4): 717-727. Here is the abstract.