A novel prokaryotic UVB photoreceptor in the cyanobacterium Chlorogloeopsis PCC 6912

We present evidence for the presence and nature of a UVB-specific photorece ptor in the cyanobacterium Chlorogloeopsis PCC 6912. The photoreceptor medi ates at least the photosensory induction of mycosporine-like amino acid (MA A) synthesis. Because MAA synthesis in this organism can also be induced un der salt stress, we could distinguish between the photosensory and the pure ly biochemical requirements of MAA synthesis. Neither visible light nor UV radiation was necessary for the biosynthetic process, thus indicating that the UVB (280-320 nm) dependence of biosynthesis is based on a UV photosenso ry capacity of the organism. An action spectrum of the MAA synthesis showed a distinct peak at 310 nm tailing down into the UVA (320-400 nm) region wi th no detected activity above 340 nm. We found that radiation below 300 nm caused significant inhibition of synthesis of MAAs indicating that the acti on spectrum at these wavelengths may not have been satisfactorily resolved. We propose that a pterin is a good candidate for a photoreceptor chromopho re as (1) reduced pterins present absorption spectra congruent with the act ion spectrum obtained; and (2) an inhibitor of the biosynthetic pathway of pterins and an antagonist of excited states of pterins, both depressed the photosensory efficiency of induction but not its chemosensory efficiency.