2 DIFFERENT ESCHERICHIA-COLI PROP PROMOTERS RESPOND TO OSMOTIC AND GROWTH-PHASE SIGNALS

proP of Escherichia coli encodes an active transport system for prolin e and glycine betaine which is activated by both hyperosmolarity and a mino acid-limited growth, proP DNA sequences far upstream from the tra nslational start site are strongly homologous to the promoter of proU, an operon that specifies another osmoregulated glycine betaine transp ort system. Mutation and deletion analysis of proP and primer extensio n experiments established that this promoter, P1, was responsible for proP's strong expression in minimal medium and its response to osmotic signals. When cells were grown in complex medium, expression from a p roP-lacZ fusion was induced three- to fourfold as growth slowed and ce lls entered stationary phase. Stationary-phase induction was dependent on rpoS, which encodes a stationary-phase sigma factor. Deletion of 1 58 bp of the untranslated leader sequence between P1 and the proP stru ctural gene abolished rpoS-dependent stationary-phase regulation. Tran scription initiation detected by primer extension within this region w as absent in an rpoS mutant, proP is therefore a member of the growing class of sigma(S)-dependent genes which respond to both stationary-ph ase and hyperosmolarity signals.