Four genes essential for bacteriochlorophyll biosynthesis were known to be encoded within a 45 kb region of the Rhodobacter sphaeroides genome, the boundaries of which are defined by puh and puf genes for reaction-centre and light-harvesting LH1 complexes. The cluster is represented by eight overlapping inserts cloned in the mobilizable vector pSUP202. We have used localized transposon Tn5 mutagenesis to characterize this cluster further; a total of 87 independent insertions were generated which identify nine genes for bacteriochlorophyll biosynthesis, six for carotenoid biosynthesis, and puhA encoding the reaction-centre H subunit. This work provides an essential framework for a detailed study of the structure and expression of genes for photosynthesis in this bacterium.

Leaf plastids of the Arabidopsis pale cress (pac) mutant do not develop beyond the initial stages of differentiation from proplastids or etioplasts and contain only low levels of chlorophylls and carotenoids. Early in development, the epidermis and mesophyll of pac leaves resemble those of wild-type plants. In later stages, mutant leaves have enlarged intercellular spaces, and the palisade layer of the mesophyll can no longer be distinguished. To study the molecular basis of this phenotype, we cloned PAC and determined that this gene is regulated by light and has the capacity to encode an acidic, predominantly alpha-helical protein. The PAC gene appears to be a novel component of a light-induced regulatory network that controls the development of leaves and chloroplasts.