FACTORS INFLUENCING THE PHYCOBILISOME ROD COMPOSITION OF THE CYANOBACTERIUM SYNECHOCOCCUS SP PCC-7942 - EFFECTS OF REDUCED PHYCOCYANIN CONTENT, LACK OF ROD-LINKERS, AND OVER-EXPRESSION OF THE ROD-TERMINATING LINKER

Four novel mutants with altered phycobilisomes were constructed in the cyanobacterium Synechococcus 7942 to study factors influencing the ro d length and composition. These mutants show (1) reduced phycocyanin c ontent, (2) reduced phycocyanin content combined with loss of the 33 k Da linker, (3) loss of the 30 kDa rod-linker and (4) overexpression of the 9 kDa rod terminating linker. For these mutants we determined the 33 to 27 kDa and 30 to 27 kDa linker ratios in the isolated phycobili somes and compared these ratios with those in the wild type. The 30 kD a linker can be incorporated into the rods in absence of the 33 kDa li nker. The incorporation of the 30 kDa linker is lower in absence of th e 33 kDa linker. When the 30 kDa linker is missing, an increase in the level of the 33 kDa linker is seen, indicating that there could be an excess of the 33 kDa linker in the cells. Our results also show that a reduction in the phycocyanin content causes a decrease in the rod le ngth simultaneously with a reduction of the 30/27 linker ratio, withou t altering the 33/27 ratio. Reduced phycocyanin content and absence of the 33 kDa linker cause a dramatic reduction in the incorporation of the 30 kDa linker into the rods in the mutant B2SMIKM. Over-expression of the 9 kDa linker results in a decreased incorporation of both the 33 and 30 kDa linkers into the rods, the effect being more pronounced for the 30 kDa linker. This result indicates that the level of the 9 k Da linker relative to those of the 33 and the 30 kDa linkers may be an important determinant of the phycobilisome rod length.