Comparison of the regulatory regions of ilvGEDA operons from several enteric organisms.

The nucleotide sequence preceding the ilvGEDA operon has been examined and compared in five enteric organisms. The sequence in Escherichia coli B was identical to the earlier-described strain K-12 sequence. The sequences of Salmonella typhimurium and Klebsiella aerogenes were remarkably similar to that of E. coli and identical in that part of the leader region that specified the putative 32-amino-acid peptide. Thus, identical secondary structures could be postulated for the leaders of all three organisms, and regulation of operon expression could be like that postulated earlier for E. coli. Different secondary structures had to be postulated for the leader transcripts of Edwardsiella tarda and Serratia marcescens. Control of attenuation of the operon in these organisms by the level of leucyl tRNA could be explained only if ribosome stalling occurred at a single leucine codon. In both organisms, that single leucine codon is the rarely used CUA rather than the CUG that is in E. coli, S. typhimurium, and K. aerogenes.