DIFFERENT STRUCTURE AND EXPRESSION OF THE OPERONS ENCODING THE MEMBRANE-BOUND HYDROGENASES FROM METHANOSARCINA-MAZEI GOL

The expression of the vho and vht operons from Methanosarcina mazei Go l, which each encode a membrane-bound hydrogenase and a cytochrome b, was analyzed under various growth conditions. Synthesis of both hydrog enases was induced at the level of transcription during methanogenesis from H-2/CO2 or methanol. Transcripts of the who operon were also det ected when Ms. mazei Gol was grown on acetate, indicating that this op eron is constitutively expressed, In contrast, mRNA from the vht opero n was not found in acetate-grown cells. Downstream of the structural g enes vhtG and vhtA and the cytochrome-b-encoding gene vhtC, an additio nal open reading frame (vhtD; 486 bp) was identified. vhtD is followed by six tandem repeats of an 11-bp sequence, which is probably a termi nation site of transcription, Northern blots revealed that vhtD is par t of the vht operon, In the who operon, a vhtD-like gene and a termina tor composed of tandem repeats could not be identified. The physiologi cal function of two genetically distinct, membrane-bound hydrogenases from Ms. mazei Gol is discussed.