The role of cyanopterin in UV/blue light signal transduction of cyanobacterium Synechocystis sp. PCC 6803 phototaxis.

We analyzed the effects of inactivating the pteridine glycosyltransferase gene (pgtA) on the photomovement of the cyanobacterium Synechocystis sp. PCC 6803 under different light conditions. The pgtA mutant displayed abnormal photomovement under UV-A/blue light. In particular, the pgtA mutant showed a negative phototactic response under UV-A (315-400 nm), whereas the wild-type did not show any photomovement. Inhibition of pterin biosynthesis by N-acetylserotonin (NAS), an inhibitor of sepiapterin reductase, also inhibited a positive phototactic response of the wild-type under white and blue light. In addition, negative phototaxis of the pgtA mutant was observed under UV-A/blue light in the presence of NAS. These results indicated that the product of the PgtA enzyme, cyanopterin, is involved in the inhibition of the negative phototaxis of the wild-type by sensing the UV-A. However, 2,4-diamino-6-hydroxypyrimidine-mediated inhibition of GTP cyclohydrolase I, the rate-limiting enzyme for pterin biosynthesis, significantly increased the positive phototaxis toward UV-A in the wild-type and the pgtA mutant. Furthermore, we measured the action spectrum of phototaxis in vivo for the wild-type and pgtA mutant. Maximal activity of the wild-type was at 300, 380 and 440 nm, indicating absorption by pterins and flavin. In particular, the UV-A/ blue peak at 380 and 440 nm obtained from the action spectrum of phototaxis was found to be closely correlated with the in vitro absorption spectrum previously reported for the cyanobacterial cryptochrome DASH. By investigating the photomovement of the wild-type and pgtA mutant to UV and blue light, we suggest that pterin can function as the chromophore of putative UV/blue photoreceptor(s) in cyanobacterial phototaxis.