Structure and Sequence of the Photosynthesis Gene Cluster

Abstract

SummaryA natural cluster of genes containing nearly all the genetic information specifically required for photosynthesis in Rhodobacter capsulatus has been sequenced. Within this 46 kilobase pair cluster are 43 identified genes which include the coding sequences for the structural reaction center polypeptides (RC), the structural light harvest I complex (LHI), as well as the coding sequences for the enzymes required for the biosynthesis of bacteriochlorophyll (BChl) and for the biosynthesis of the carotenoids, spheroidene and spheroidenone.This cluster and the accompanying DNA sequence have been extremely valuable in the identification of DNA sequences in other organisms, both bacteria and green plants, with comparable functions associated with photosynthesis. These sequence homologies have been tabulated. In addition, these homologies provide evolutionary information relating to reaction centers, light harvesting complexes, carotenoid biosynthesis and chlorophyll biosynthesis. In particular, a strong structural similarity has been identified between some reductive enzymes in the chlorophyll biosynthetic pathways and nitrogen reductases associated with nitrogen fixation. This has led to a new interpretation of Granick’s hypothesis, the hypothesis that steps in biosynthetic pathways can be assumed to be linearly associated with the order of evolution of the corresponding activity in time.Finally, the Rhodobacter capsulatus photosynthetic gene cluster and the corresponding sequence will clearly be the source of genes for recombinant constructs, for mutational studies on reaction centers and light harvesting complexes and for the overproduction ofenzymatic activities essential for mechanistic biochemistry and for structural studies for many years to come.