The effects of oxygen and light on the expression of the puf operon were investigated in Roseobacter denitrificans in a comparison with those in Rhodobacter sphaeroides. In darkness, the levels of the total puf mRNA in Ros. denitrificans were about 1.3 times those in Rb. sphaeroides at low concentrations of oxygen, reflecting the accumulation of bacteriochlorophyll and carotenoids. The oxygen tension, up to 94% saturation of dissolved oxygen, did not affect the levels of the total puf transcripts in Ros. denitrificans, whereas those in Rb. sphaeroides were reduced to 55% of the maximum level even at 50% saturation. Four puf-specific transcripts were detected: a 0.5-kb transcript was the most abundant; 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 the 0.5-kb transcript encoded the LHI-alpha and LHI-beta subunits (pufBA), the 1.2-kb transcript encoded puf-BA and part of pufL, and the 1.9-kb 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 structures at the 3' terminus of the 0.5-kb transcript which acted as terminators of transcription and, possibly, as protection against nucleolytic digestion. Light inhibited the expression of the puf operon in Ros. denitrificans more effectively than that in Rb. sphaeroides. The insensitivity to oxygen, as well as the sensitivity to light, of the expression of the puf operon in Ros. denitrificans, which was different from that in Rb. sphaeroides, seemed to represent a mode of adaptation that allowed the former cells to avoid photodynamic damage by light under highly aerobic conditions.