STRUCTURE AND REGULATION OF THE SALMONELLA-TYPHIMURIUM RNC-ERA-RECO OPERON

The Escherichia coli rnc-era-recO operon encodes ribonuclease III (RNa se III; a dsRNA endonuclease involved in rRNA and mRNA processing and decay), Era (an essential G-protein of unknown function) and RecO (inv olved in the RecF homologous recombination pathway). Expression of the me and era genes is negatively autoregulated: RNase III cleaves the r ncO 'operator' in the untranslated leader, destabilizing the operon mR NA. As part of a larger effort to understand RNase III and Era structu re and function, we characterized rnc operon structure, function and r egulation in the closely related bacterium Salmonella typhimurium. Con struction of a S typhimurium strain conditionally defective for RNase III and Era expression showed that Era is essential for cell growth. T his mutant strain also enabled selection of recombinant clones contain ing the intact S typhimurium mc-era-recO operon, whose nucleotide sequ ence, predicted protein sequence, and predicted rncO RNA secondary str ucture were all highly conserved with those of E coli. Furthermore, ge netic and biochemical analysis revealed that S typhimurium rnc gene ex pression is negatively autoregulated by a mechanism very similar or id entical to that in E coli, and that the cleavage specificities of RNas e IIIS.t. and RNase IIIE.c. are indistinguishable with regard to rncO cleavage and S typhimurium 23S rRNA fragmentation in vivo.