PARAQUAT REGULATION OF HMP (FLAVOBEMOGLOBIN) GENE-EXPRESSION IN ESCHERICHIA-COLI K-12 IS SOXRS INDEPENDENT BUT MODULATED BY SIGMA(S)

We report the first example of a gene, hmp, encoding a soluble flavohe moglobin in Escherichia coli K-12, which is up-regulated by paraquat i n a SoxRS-independent manner. Unlike what is found for other paraquat- inducible genes, high concentrations of paraquat (200 mu M) were requi red to increase the level of hmp expression, and maximal induction was observed only after 20 min of exposure to paraquat, Neither a mutatio n in soxS nor one in soxR prevented the paraquat-dependent increase in Phi(hmp-lacZ) expression, but either mutant allele delayed full expre ssion of Phi(hmp-lacZ) activity after paraquat addition, Induction of hmp by paraquat was demonstrated in aerobically grown cultures during exponential growth and the stationary phase, thus revealing two Sox-in dependent regulatory mechanisms. Induction of hmp by paraquat in the s tationary phase was dependent on the global regulator of stationary-ph ase gene expression, RpoS (sigma(s)), However, a mutation in rpoS did not prevent an increase in hmp expression by paraquat in exponentially growing cells, Induction of sigma(s) in the exponential phase by heat shock also induced Phi(hmp-lacZ) expression by the presence of paraqu at, supporting the role of sigma(s) in one of the regulatory mechanism s. Mutations in oxyR or rob, known regulators of several stress promot ers in E. coli, had no effect on the induction of hmp by paraquat, Oth er known superoxide-generating agents (plumbagin, menadione, and phena zine methosulfate) were not effective in inducing hmp expression.