CONSEQUENCES FOR THE ORGANIZATION OF REACTION CENTER-LIGHT-HARVESTING-ANTENNA-1 (LH1) CORE COMPLEXES OF RHODOBACTER-SPHAEROIDES ARISING FROM DELETION OF AMINO-ACID-RESIDUES FROM THE C-TERMINUS OF THE LH1-ALPHA-POLYPEPTIDE

The light harvesting antenna 1 (LH1) complex of Rhodobacter sphaeroide s is intimately associated with the reaction center (RC) as part of th e reaction center RC-LH1 core complex, The pufA gene has been modified such that between 5 and 16 amino acid residues were progressively del eted from the C terminus of the LH1 alpha polypeptide. The two largest deletions produced strains which were deficient in LH1, The remaining four deletion mutants exhibited significant reductions in the average level of LH1 per reaction center, Analysis of detergent-solubilized c ores on sucrose gradients showed that the mutant strains had a sizeabl e population of antenna-deficient reaction centers in addition to core complexes with a reduced ratio of LH1:RC. The decrease in the ratio o f LH1:RC in core complexes of the mutant strains was accompanied by a progressive blue shift of the absorbance maximum of LH1, which we attr ibute to the reduced aggregation state of LH1 in the smaller cores, Th e PufX polypeptide was not required for photosynthetic growth in mutan ts with reduced core sizes, We conclude that the level of LH1 in the b acterial membrane, and the aggregation state of LH1 in core complexes, are both influenced by the C terminus of the alpha polypeptide, and w e discuss possible models for the organization of the core complex in Rb. sphaeroides.