Directed Mutations Affecting the Putative Bacteriochlorophyll‐Binding Sites in the Light‐Harvesting I Antenna of <i>Rhodobacter capsulatus</i>

Abstract

Abstract Oligonucleotide‐mediated mutagenesis of the light‐harvesting I (LHI; B880 complex) antenna genes from Rhodobacter capsulatus has been used to study the bacteriochlorophyll‐peptide interactions in this photosynthetic membrane complex. A system of deletion strains and complementing plasmids has been used to assay the effects of changing single amino acid residues in the a structural polypeptide of LHI. Fourteen mutations have been introduced in the putative bacteriochlorophyll‐binding site: Ala‐X‐X‐X‐His. Residues with side chains smaller than valine can substitute for alanine, but the LHI complex is lost from the membrane if alanine is replaced by residues larger than valine, or if histidine is replaced by arginine, asparagine, aspartic acid, glutamine, proline, or threonine. The block in biogenesis is not transcriptional, since the associated reaction center, whose genes are known to be downstream from the LHI genes, continues to be inserted into the membrane. The membrane‐bound reaction center is photochemically active in these LHI‐depleted strains. Such strains may be useful for directly observing electron transfer properties of the reaction center in the membrane. This system is a model for studying the assembly of integral membrane proteins, since easily assigned optical transitions of the prosthetic groups signal proper assembly.