THE HIGHLY ABUNDANT CHLOROPHYLL-PROTEIN COMPLEX OF IRON-DEFICIENT SYNECHOCOCCUS SP PCC7942 (CP43) IS ENCODED BY THE ISIA GENE

A chlorophyll (Chl)-protein complex designated CPVI-4 becomes the majo r pigment-protein complex in Synechococcus sp. PCC7942 cells grown und er conditions of iron limitation. Work by Laudenbach et al. (J Bacteri ol [1988] 170: 5018-5026) has identified an iron-repressible operon, d esignated isiAB, containing the flavodoxin gene and a gene predicted t o encode a Chi-binding protein resembling CP43 of photosystem II. To t est the hypothesis that the CP43-like protein is a component of the CP VI-4 complex, we have inactivated the isiAB operon in Synechococcus sp . PCC7942 using directed insertional mutagenesis. Mutant cells grown u nder conditions of iron limitation exhibit pronounced changes in their spectroscopic and photosynthetic properties relative to similarly gro wn wild-type cells. Notably, the strong 77 K fluorescence emission at 685 nm, which dominates the spectrum of iron-deficient wild-type cells , is dramatically reduced in the mutant. The loss of this emission app ears to unmask the otherwise obscured photosystem II emissions at 685 and 695 nm. Most importantly, mildly denaturing gel electrophoresis sh ows that mutant cells no longer express the CPVI-4 complex, indicating that the isiA gene encodes a component of this abundant Chl-protein c omplex.