Altered Pterin Patterns in Photoreceptor Mutants of Phycomyces blakesleeanus with Defective madl Gene

Abstract

AbstractAction spectra of photogravitropic equilibrium were measured for the wild type of the lower fungus Phycomyces blakesleeanus and three photobehavioral mutants with defects in the madl gene. The action spectrum for the wild type NRRL1555 had major peaks at 383 and near 460 nm and subsidiary peaks at 365 and 422 nm. The action spectra of the mutants, L1 49 mad1712, L151 mad1714 and L153 mad1716 differed significantly from that of the wild type. One prominent feature of the three mutants was hat the near‐UV peaks at 365 and 383 nm, which were not well resolved in the wild type, were of approximately equal height in the mutants and were separated by an extremely sharp valley at 378 nm. The steepness of this valley suggests interaction of multiple receptors. The second prominent feature of the mutants was their enhanced 422 nm peak. The gross changes of the photogravitropic action spectra associated with the madl genotype indicate that the respective gene product acts early in the photosensory transduction chain, very likely at the level of a complex photoreceptor system. Flavins and pterins, two pigment classes which were expected to function as chromophores of the near‐UV/blue light photoreceptor system, were analyzed for stage I sporangiophores of the wild‐type and the mutant strains by HPLC with fluorescence detection. In the wild‐type strain NRRL1555, and also in the three madl mutants, flavins were found to be present at the following concentrations: riboflavin (2.9 × 10−6 M), FMN (3.8 × 10−6M) and FAD (1.3 × 10−6 M). No significant effect of the madl mutations on the flavin content could be discerned. Among the pterins found in the wild type and the madl mutants were biopterin, 6,7‐dimethylpterin, neopterin, pterin and xanthopterin. These pterins occurred in all strains in the micromolar range and none of them was significantly reduced in the mutants. However, biopterin, 6,7‐dimethylpterin and xanthopterin occurred in some excess in one of the madl mutants. The most significant feature of the madl mutants was that they had almost completely lost one unidentified pterin with a retention time of 18 − 20 min. Another two unidentified pterins were reduced about twofold in the mutants compared to the wild type. The results suggest an involvement of pterins in the photoreception of near‐UV and blue light in Phycomyces.