Oxygen inhibition of globin gene transcription and bacterial haemoglobin synthesis in Vitreoscilla

A soluble dimeric haemoprotein, structurally and functionally similar to plant and animal haemoglobins, is found in the Gram-negative aerobic bacterium Vitreoscilla sp., strain C1. Vitreoscilla haemoglobin (VtHb) increases in concentration when the cells are exposed to hypoxic conditions. The globin part of VtHb is encoded by a single gene (vgb). An RNA transcript, approximately 500 bases long, specific for vgb was detected after Northern hybridization. The relative amount of this mRNA increased in cells grown at low levels of oxygen. Two enzymes important for haemoglobin function are delta-aminolaevulinic acid synthase (ALAS), which is necessary for haem biosynthesis, and NADH-methaemoglobin reductase, which is necessary to keep VtHb in the physiologically functional ferrous state. An increase in ALAS specific activity under hypoxic conditions preceded the increased haem production. Cellular reductase content also increased when the VtHb increased in cells grown under hypoxic conditions. The ratio of cellular reductase activity to VtHb content remained relatively constant in cells grown under a variety of conditions. The data suggest that in Vitreoscilla the transcription of the globin gene and the biosynthesis of two enzymes important for VtHb function are regulated by oxygen.