SUBUNIT INTERACTIONS AND PROTEIN STABILITY IN THE CYANOBACTERIAL LIGHT-HARVESTING PROTEINS

Strain 4R is a phycocyanin-minus mutant of the unicellular cyanobacter ium Synechocystis sp, strain 6803. Although it lacks the light-harvest ing protein phycocyanin, 4R has normal levels of phycocyanin (cpc) tra nscripts. Sequence analysis of the cpcB gene encoding the phycocyanin beta subunit shows an insertion mutation in 4R that causes early termi nation of translation. Other work has shown that the phycocyanin alpha subunit and the linker proteins encoded on the cpc transcripts are al l functional in 4R, yet the defective phycocyanin beta subunit results in the complete absence of the alpha subunit and the linkers. Phycocy anin-minus mutants were constructed in a wild-type background by inter ruption of cpcB and cpcA with an antibiotic resistance gene and were c ompared with the 4R strain. Immunoblot analysis of the mutants demonst rated that interruption of one subunit was accompanied by a complete a bsence of the unassembled partner subunit. Phycocyanin assembly begins with the formation of the alpha beta heterodimer (the monomer) and co ntinues through higher-order trimeric and hexameric aggregates th;tt a ssociate with linker proteins to form the phycobilisome rods. The resu lts in this paper indicate that monomer formation is a critical stage in the biliprotein assembly pathway and that unassembled subunits are subject to stringent controls that prevent their appearance in vivo.