Expression and regulation of a silent operon, hyf, coding for hydrogenase 4 isoenzyme in Escherichia coli

On the basis of hyf-lacZ fusion studies, the hyf operon of Escherichia coli, noted for encoding the fourth hydrogenase isoenzyme (HYD4), is not expressed at a significant level in a wild-type strain. However, mutant FhlA proteins (constitutive activators of the hyc-encoded hydrogenase 3 isoenzyme) activated hyf-lacZ. HYfR, an FhlA homolog encoded by the hyfR gene present at the end of the hyf operon, also activated transcription of hyf-lacZ but did so only when hyfR was expressed from a heterologous promoter. The HYD4 isoenzyme did not substitute for HYD3 in H-2 production. Optimum expression of hyf-lacZ required the presence of cyclic AMP receptor protein-cyclic AMP complex and anaerobic conditions when HyfR was the activator.