The high light-inducible polypeptides in Synechocystis PCC6803 -: Expression and function in high light

There are five Synechocystis PCC6803 genes encoding polypeptides with similarity to the Lhc polypeptides of plants. Four of the polypeptides, designated HliA-D (Dolganov, N. A M., Bhaya, D., and Grossman, A. R. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 636-640) (corresponding to ScpC, ScpD, ScpB, and ScpE in Funk, C., and Vermaas, W. (1999) Biochemistry 38, 9397-9404) contain a single transmembrane domain. The fifth polypeptide (HemH) represents a fusion between a ferrochelatase and an Hli-like polypeptide. By using an epitope tag to identify specifically the different Hli polypeptides, the accumulation of each (excluding HemH) was examined under various environmental conditions. The levels of all of the Hli polypeptides were elevated in high light and during nitrogen limitation, whereas HliA, HliB, and HliC also accumulated to high levels following exposure to sulfur deprivation and low temperature. The temporal pattern of accumulation was significantly different among the different Hli polypeptides. HliC rapidly accumulated in high light, and its level remained high for at least 24 h. HliA and HliB also accumulated rapidly, but their levels began to decline 9-12 h following the imposition of high light. HliD was transiently expressed in high light and was not detected 24 h after the initiation of high light exposure. These results demonstrate that there is specificity to the accumulation of the Hli polypeptides under a diverse range of environmental conditions. Furthermore, mutants for the individual and combinations of the hli genes were evaluated for their fitness to grow in high light. Although all of the mutants grew as fast as wild-type cells in low light, strains inactivated for hliA or hliC/hliD were unable to compete with wild-type cells during co-cultivation in high light. A mutant lacking all four hli genes gradually lost its photosynthesis capacity and died in high light. Hence, the Hli polypeptides are critical for survival when Synechocystis PCC6803 is absorbing excess excitation energy and may allow the cells to cope more effectively with the production of reactive oxygen species.