Transcriptional regulation of the proC gene of Salmonella typhimurium

We found that the expression of beta-galactosidase in Salmonella typhimurium strains carrying proC-lacZ fusions was neither repressed by excess proline nor derepressed by proline limitation. Except for a three- to fourfold decrease in the beta-galactosidase specific activity under conditions causing a severely reduced growth rate, the expression of the proC-lacZ fusions was nearly invariant under a variety of culture conditions. Thus, the proC gene is unlike most other amino acid biosynthetic genes in that its expression is nearly constitutive.