Conan S.

ORF1696 is included as part of the bchFNBHLMORFl 696puhA superoperon found within the photosynthesis gene cluster on the chromosome of Rhodobacter capsulatus and mutation of ORF1696 results in reduced levels of the light-harvesting I (LHI) complex in cells grown photosynthetically. ORF1696 mutant strains were created by insertion of antibiotic resistance cartridges at different sites within the ORF1696 gene in a strain that lacks the lightharvesting II (LHII) complex. All of the mutant strains were deficient in the LHI complex, including one (ΔNae) 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 (ΔMun) with a novel absorption spectrum. The two more 3' mutant strains (ΔStu and ΔNae) were restored to levels close to the parental strain phenotype when frans-complemented with a plasmid expressing the ORF1696 gene, but ΔMun was not. The absorption spectrum of ΔMun resembled that of a strain which had a polar mutation in ORF1696. A comparison of LHI complex assembly kinetics showed that assembly occurred 2.8-fold faster in the parental strain compared to strain ΔStu. In contrast, LHI complex decay occurred 1.7-fold faster in the ORF1696 parental strain than in ΔStu. These results indicate that the major activity of the ORF1696 protein is in LHI complex assembly. A membrane topology model of the ORF1696 protein is proposed consisting of twelve membrane-spanning domains with both the N - and C-termini localized to the cytoplasmic side of the membrane similar to that predicted for the PucC protein of R. capsulatus which is required to maintain levels of the LHII complex. Suggestions for the function of the ORF1696 protein in LHI complex assembly are also proposed.Science, Faculty ofMicrobiology and Immunology, Department ofGraduat