Positive transcriptional feedback controls hydrogenase expression in Alcaligenes eutrophus H16

The protein HoxA is the central regulator of the Alcaligenes eutrophus H16 hox regulon, which encodes two hydrogenases, a nickel permease and several accessory proteins required for hydrogenase biosynthesis. Expression of the regulatory gene hoxA was analyzed. Screening of an 8-kb region upstream of hoxA with a promoter probe vector localized four promoter activities. One of these was found in the region immediately 5' of hoxA; the others were co rrelated with the nickel metabolism genes hypA1, hypB1, and hypX. All four activities were independent of HoxA and of the minor transcription factor s igma(54). Translational fusions revealed that hoxA is expressed constitutiv ely at low levels. In contrast to these findings, immunoblotting studies re vealed a clear fluctuation in the HoxA pool in response to conditions which induce the hox regulon. Quantitative transcript assays indicated elevated levels of hyp mRNA under hydrogenase-derepressing conditions. Using interpo son mutagenesis, me showed that the activity of a remote promoter is requir ed for hydrogenase expression and autotrophic growth. Site-directed mutagen esis revealed that P-MBH, which directs transcription of the structural gen es of the membrane-bound hydrogenase, contributes to the expression of hoxA under hydrogenase-derepressing conditions. Thus, expression of the hox reg ulon is governed by a positive feedback loop mediating amplification of the regulator HoxA. These results imply the existence of an unusually large (c a. 17,000-nucleotide) transcript.