GENETIC COMPLEMENTATION AND KINETIC ANALYSES OF RHODOBACTER-CAPSULATUS ORF1696 MUTANTS INDICATE THAT THE ORF1696 PROTEIN ENHANCES ASSEMBLY OF THE LIGHT-HARVESTING-I COMPLEX

Rhodobacter capsulatus ORF1696 mutant strains were created by insertio n of antibiotic resistance cartridges at different sites within the OR F1696 gene in a strain that lacks the light-harvesting II (LHII) compl ex. Steady-state absorption spectroscopy profiles and the kinetics of the light-harvesting I (LHI) complex assembly and decay were used to e valuate the function of the ORF1696 protein in various strains. All of the mutant strains were found to be deficient in the LHI complex, inc luding one (Delta Nae) with a disruption located 13 codons before the 3' end of the gene. A 5'-proximal disruption after the 31st codon of O RF1696 resulted in a mutant strain (Delta Mun) with a novel absorption spectrum. The two strains with more 3' disruptions (Delta Stu and Del ta Nae) were restored nearly to the parental strain phenotype when tra ns complemented with a plasmid expressing the ORF1696 gene, but Delta Mun was not. The absorption spectrum of Delta Mun resembled that of a strain which had a polar mutation in ORF1696. We suggest that a rho-de pendent transcription termination site exists between the MunI and pro ximal StuI sites of ORF1696. A comparison of LHI complex assembly kine tics showed that assembly occurred 2.6-fold faster in the parental str ain than in strain Delta Stu. In contrast, LHI complex decay occurred 1.7-fold faster in the ORF1696 parental strain than in Delta Stu. Thes e results indicate that the ORF1696 protein has a major effect an LHI complex assembly, and models of ORF1696 function are proposed.