Genetic Manipulation of the Antenna Complexes of Purple Bacteria

Abstract

Photosynthetic bacteria have relatively simple photosystems, and they are therefore attractive models for the study of the absorption of light, and the transfer and trapping of excitation energy. This simplicity extends to the organization and expression of genetic information, and it would be a missed opportunity if molecular genetics were not used to examine the assembly, organization and function of light harvesting complexes. Further more, directed mutagenesis ofgenes encoding structural subunits of the antenna, in combination with various forms of spectroscopy, provides a means of testing structural models for these complexes, and of enhancing our understanding of existing crystallographic data. The potential of using genetic techniques to provide partial photosystems was realized over thirty years ago by, among others, Clayton and Sistrom, and indeed the carotenoidless, LH2- deficient mutant R26 isolated by Clayton in 1961 is still used today. The availability of mutants lacking either the peripheral LH2 complex or the core LH1 complex provides a means of simplifying spectroscopic studies that would otherwise be complicated by overlapping signals, as well as a way of rapidly and efficiently obtaining a given complex using detergent-based isolation procedures. This chapter provides an account of the way in which various antenna mutants have been used for such purposes, concentrating on the bacteria for which an array of molecular genetic tools have been developed, Rhodobacter (Rb.) capsulatus and Rb. sphaeroides. The latter part of the chapter concentrates on more recent developments in which protein engineering has been used to modify the assembly and the spectroscopic properties of LH1 and LH2, by altering various aminoacids singly, or in groups. Further progress in this rapidly moving field is aided by new developments in spectroscopy, as well as by the application of combinatorial mutagenesis and the heterologous expression of LH genes. It is expected that molecular genetics will play an increasingly important role in the study of light harvesting complexes.