Genetic complementation and kinetic analyses of Rhodobacter capsulatus ORF1696 mutants indicate that the ORF1696 protein enhances assembly of the light-harvesting I complex.

Abstract

Rhodobacter capsulatus ORF1696 mutant strains were created by insertion of antibiotic resistance cartridges at different sites within the ORF1696 gene in a strain that lacks the light-harvesting II (LHII) complex. Steady-state absorption spectroscopy profiles and the kinetics of the light-harvesting I (LHI) complex assembly and decay were used to evaluate the function of the ORF1696 protein in various strains. All of the mutant strains were found to be deficient in the LHI complex, including one (deltaNae) with a disruption located 13 codons before the 3' end of the gene. A 5'-proximal disruption after the 31st codon of ORF1696 resulted in a mutant strain (deltaMun) with a novel absorption spectrum. The two strains with more 3' disruptions (deltaStu and deltaNae) were restored nearly to the parental strain phenotype when trans complemented with a plasmid expressing the ORF1696 gene, but deltaMun was not. The absorption spectrum of deltaMun resembled that of a strain which had a polar mutation in ORF1696. We suggest that a rho-dependent transcription termination site exists between the MunI and proximal StuI sites of ORF1696. A comparison of LHI complex assembly kinetics showed that assembly occurred 2.6-fold faster in the parental strain than in strain deltaStu. In contrast, LHI complex decay occurred 1.7-fold faster in the ORF1696 parental strain than in deltaStu. These results indicate that the ORF1696 protein has a major effect on LHI complex assembly, and models of ORF1696 function are proposed.