DELAY OF CELL-DIFFERENTIATION IN ANABAENA-AEQUALIS CAUSED BY UV-B RADIATION AND THE ROLE OF PHOTOREACTIVATION AND EXCISION-REPAIR

The effect of ultraviolet light on cell differentiation was studied in the cyanobacterium Anabaena aequalis. Exposure of cells to UV-B wavel engths (280-320 nm) significantly delayed the differentiation of veget ative cells into heterocysts and akinetes at doses up to 56 kJ m(-2). Heterocyst differentiation was essentially stopped at all exposure lev els when photoreactivation was prevented, even when excision repair wa s available to the cells. Photoreactivated samples produced heterocyst s at doses through 28 kJ m(-2), after which differentiation dropped st eeply to near zero levels. Some recovery of differentiation was eviden t at higher doses but at levels much below that of controls. Akinete d ifferentiation was only slightly delayed by the exposures when cells w ere photoreactivated. Samples then showed rapid differentiation with t he numbers of akinetes significantly greater than controls. Cells that did not receive photoreactivating light showed a greater initial dela y in differentiation but 2 weeks after the exposures had recovered to control levels. Caffeine had more effect on the differentiation of aki netes than heterocysts. Inhibition of excision repair greatly reduced differentiation in photoreactivated samples and essentially eliminated differentiation in the nonphotoreactivated samples.