EVIDENCE FOR INVOLVEMENT OF PROTEINS HU AND RPOS IN TRANSCRIPTION OF THE OSMORESPONSIVE PROU OPERON IN ESCHERICHIA-COLI

Transcription of the proU operon of Escherichia coli is induced severa l hundred-fold upon growth at elevated osmolarity, but the underlying mechanisms are incompletely understood. Three cis elements appear to a ct additively to mediate proU osmoresponsivity: (i) sequences around a promoter, P1, which is situated 250 bp upstream of the first structur al gene proV; (ii) sequences around another (sigma(70)-dependent) prom oter, P2, which is situated 60 bp upstream of proV; and (iii) a negati ve regulatory element present within the proV coding region. These thr ee cis elements are designated, respectively, P1R, P2R, and NRE. trans -acting mutants with partially derepressed proU expression have been o btained earlier, and a vast majority of the mutations affect the gene encoding the nucleoid protein HNS. In this study we employed a selecti on for trans-acting mutants with reduced proU(+) expression, and we ob tained a derivative that had suffered mutations in two separate loci d esignated dpeA and dpeB. The dpeB mutation caused a marked reduction i n promoter P1 expression and was allelic to rpoS, the structural gene for the stationary-phase-specific sigma factor of RNA polymerase. Expr ession from P1 was markedly induced, in an RpoS-dependent manner, in s tationary-phase cultures. In contrast to the behavior of the isolated P1 promoter, transcription from a construct carrying the entire proU c is-regulatory region (P1R plus P2R plus NRE) was not significantly aff ected by either growth phase or RpoS. The dpeA locus was allelic to hu pB, which along with hupA encodes the nucleoid protein HU. hupA hupB d ouble mutants exhibited a pronounced reduction in proU osmotic inducib ility. BU appears to affect proU regulation through the P2R mechanism, whereas the effect of HNS is mediated through the NRE.