Proline Utilization by Bacillus subtilis: Uptake and Catabolism

l -Proline can be used byBacillus subtilis as a sole source of carbon or nitrogen. We tracedl -proline utilization genetically to theputBCP (ycgMNO ) locus. TheputBCP gene cluster encodes a high-affinity proline transporter (PutP) and two enzymes, the proline dehydrogenase PutB and the Δ1 -pyrroline-5-carboxylate dehydrogenase PutC, which jointly catabolizel -proline tol -glutamate. Northern blotting, primer extension, andputB-treA reporter gene fusion analysis showed that theputBCP locus is transcribed as anl -proline-inducible operon. Its expression was mediated by a SigA-type promoter and was dependent on the proline-responsive PutR activator protein. Induction ofputBCP expression was triggered by the presence of submillimolar concentrations ofl -proline in the growth medium. However, the very large quantities ofl -proline (up to several hundred millimolar) synthesized byB. subtilis as a stress protectant against high osmolarity did not induceputBCP transcription. Induction ofputBCP transcription by externall -proline was not dependent onl -proline uptake via the substrate-inducible PutP or the osmotically inducible OpuE transporter. It was also not dependent on the chemoreceptor protein McpC required for chemotaxis towardl -proline. Our findings imply thatB. subtilis can distinguish externally suppliedl -proline from internall -proline pools generated throughde novo synthesis. The molecular basis of this regulatory phenomenon is not understood. However, it provides theB. subtilis cell with a means to avoid a futile cycle ofde novo l -proline synthesis and consumption by not triggering the expression of theputBCP l -proline catabolic genes in response to the osmoadaptive production of the compatible solutel -proline.