Regulation of proline utilization in enteric bacteria: cloning and characterization of the Klebsiella put control region.

Enteric bacteria can grow on proline as the sole nitrogen and carbon source. Expression of the proline utilization (put) operon in Klebsiella strains and Escherichia coli is responsive to nitrogen regulation. In contrast, Salmonella typhimurium cannot activate put operon expression when growing in medium with glucose as a carbon source and proline as the sole nitrogen source. To compare nitrogen regulatory sites in the control regions of the put operons in these three closely related genera, we cloned the Klebsiella put operon onto a plasmid. The putA and putP genes were localized on the plasmid by transposon mutagenesis. The DNA sequence of the put control region was determined and compared with those of the put control regions from S. typhimurium and E. coli. The overall size and organization of the put control region were very similar in all three bacteria. However, no obvious ntr regulatory sites were found in this region, and transcription of the put genes started at the same sites during growth with limiting or excess nitrogen. These results strongly suggested that the Klebsiella put operon may not be directly regulated by the ntr system.