THE PUTA GENE OF AGROBACTERIUM-TUMEFACIENS IS TRANSCRIPTIONALLY ACTIVATED IN RESPONSE TO PROLINE BY AN LRP-LIKE PROTEIN AND IS NOT AUTOREGULATED

The Agrobacterium tumefaciens putA gene, which encodes proline dehydro genase, is transcriptionally induced by exogenous proline. In contrast to the putA genes of enteric bacteria, the A. tumefaciens putA gene i s not regulated by the PutA protein, as the putA promoter remained str ongly proline inducible in strains lacking PutA, A putA null mutation increased the expression of the putA promoter under a variety of condi tions, However, this mutation is predicted to increase the cytoplasmic concentration of proline, and this alone probably accounts for its ef fects on putA expression. The putA promoter was also strongly induced by valine, and the putA genotype did not affect expression by this gra tuitous inducer, An open reading frame (ORF) encoding an Lrp-like prot ein was found transcribed divergently from putA. Disruption of this OR F, designated putR, abolished induction of the putA promoter by prolin e or valine. In addition to activating putA, PutR also repressed its o wn transcription, and this autorepression was only slightly affected b y exogenous proline, The transcription start sites for the putA and pu tR genes are separated by 64 nucleotides, suggesting that PutR could r egulate both promoters by binding to a single operator.