Ultraviolet and osmotic stresses induce and regulate the synthesis of mycosporines in the cyanobacterium Chlorogloeopsis PCC 6912

The cyanobacterium Chlorogloeopsis PCC 6912 was found to synthesize and acc umulate two putative UV sunscreen compounds of the mycosporine (mycosporine -like amino acid; MAA) type: mycosporine-glycine and shinorine. These MAAs were not constitutively present in the cells; their synthesis could be indu ced specifically either by exposure to UVB radiation (280-320 nm) or by osm otic stress, but not by other stress factors such as heat or cold shock, nu trient limitation or photooxidative stress. A significant synergistic enhan cement of MAA synthesis was observed when both stress factors were applied in combination. Although osmotic stress could induce MAA syn thesis, compar ison of the intracellular contents of MAAs with those of sugar osmolytes (g lucose and trehalose) indicated that MAAs play no significant role in attai ning osmotic homeostasis. UVB strongly enhanced the accumulation of shinori ne, whereas osmotic stress had a more pronounced effect on mycosporine-glyc ine. This differential effect on the steady-state contents of each MAA coul d be explained either by differential regulation of biosynthesis or by diff erential loss rates of MAAs (leakage) under each condition. A preferential leakage of mycosporine-glycine from the cells after a hypoosmotic shock was detected. The results are interpreted in terms of an adaptive necessity fo r a combined regulatory control responding to both UV and external osmotic conditions in organisms that accumulate water-soluble sunscreens intracellu larly.