EXPRESSION OF THE PUF OPERON IN AN AEROBIC PHOTOSYNTHETIC BACTERIUM, ROSEOBACTER-DENITRIFICANS

The effects of oxygen and light on the expression of the puf operon we re investigated in Roseobacter denitrificans in a comparison with thos e in Rhodobacter sphaeroides. In darkness, the levels of the total puf mRNA in Ros.denitrificans were about 1.3 times those in Rb.sphaeroide s at low concentrations of oxygen, reflecting the accumulation of bact eriochlorophyll and carotenoids. The oxygen tension, up to 94% saturat ion of dissolved oxygen, did not affect the levels of the total puf tr anscripts in Ros.denitrificans, whereas those in Rb.sphaeroides were r educed to 55% of the maximum level even at 50% saturation. Four puf-sp ecific transcripts were detected: a 0.5-kb transcript was the most abu ndant; 1.2-kb and 1.9-kb transcripts accumulated at low levels; and a 3.5-kb transcript accumulated at very low levels under all conditions tested. The levels of the individual transcripts were barely affected by molecular oxygen, An S-1 nuclease protection assay revealed that th e 0.5-kb transcript encoded the LHI-alpha and LHI-beta subunits (pufBA ), the 1.2-kb transcript encoded pufBA and part of pufL, and the 1.9-k b transcript encoded pufBAL and part of pufM. It was not clear whether the 3.5-kb transcript encoded the entire pufBALM and the gene for the polypeptide moiety of cytochrome c, The difference in levels between the 0.5-kb transcript and the other transcripts (1.2 kb, 1.9 kb, and 3 .5 kb) was presumed to be due to the presence of several stem-loop str uctures at the 3' terminus of the 0.5-kb transcript which acted as ter minators of transcription and, possibly, as protection against nucleol ytic digestion. Light inhibited the expression of the puf operon in Re s. denitrificans more effectively than that in Rb. sphaeroides, The in sensitivity to oxygen, as well as the sensitivity to light, of the exp ression of the puf operon in Ros. denitrificans, which was different f rom that in Rb.sphaeroides, seemed to represent a mode of adaptation t hat allowed the former cells to avoid photodynamic damage by light und er highly aerobic conditions.