Stimulated biosynthesis of flavins in Photobacterium phosphoreum IFO 13896 and the presence of complete rib operons in two species of luminous bacteria.

Photobacterium phosphoreum IFO 13896 emits light strongly when cultured in medium containing 3% NaCl, but only weakly in medium containing 1% NaCl. It is known that dim or dark mutants appear frequently and spontaneously from this parent strain. To confirm that riboflavin biosynthesis is stimulated when the lux operon is active, the amount of light emitted and flavins synthesized under strongly or weakly light emitting conditions was determined. In comparison with the parent strain cultured in 3% NaCl, the same strain cultured in 1% NaCl emitted 1/36 the light and produced 1/4 the flavins, while three dim or dark mutants, M1, M2 and M3 cultured in 3% NaCl, emitted almost no light, 1/58 the light and 1/10 the light and produced 1/8, 1/5 and 1/3 the amount of flavins, respectively. From these results, we deduced that the genes for riboflavin synthesis, rib genes, are organized in an operon in this strain. In P. phosphoreum NCMB 844, it has been reported that a rib gene cluster is present just downstream of the lux operon. However, among rib genes, the gene for pyrimidine deaminase/pyrimidine reductase, ribD, was not found in this cluster. Because a complete rib operon seems to be necessary for efficient regulation at the transcriptional level, we expected ribD to be present downstream of this cluster and sequenced this region, using SUGDAT, Sequencing Using Genomic DNA As a Template. We could not find this gene but found a gene for hybrid-cluster protein (prismane protein). To find ribD in a different region, a partial ribD sequence was amplified and sequenced using a PCR-based method, and subsequently the genomic DNA was sequenced in both directions from this partial sequence using SUGDAT. Because ribC was found just downstream of ribD, we sequenced further downstream of ribC and confirmed that another complete set of rib genes, ribD, ribC, ribBA, and ribE, is present in P. phosphoreum. The presence of a complete rib operon in P. phosphoreum explains why this species can synthesize flavins at enhanced levels to sustain a strong light emission. Furthermore, we sequenced the rib operon in Vibrio fischeri, another representative luminous bacterium, in a manner similar to that described above, and confirmed that a complete operon is present also in this species. The organization of rib genes in an operon in the Proteobacteria γ-subdivision is discussed.