The gene crtI mediates the conversion of phytoene into colored carotenoids in Rhodopseudomonas capsulata.

G Giuliano,D Pollock,P A Scolnik

doi:10.1016/s0021-9258(18)69250-6

Abstract

Carotenoids are membrane pigments present in all photosynthetic organisms, providing essential photoprotective functions. The first carotenoid formed in the pathway is phytoene, a colorless compound which is then converted into colored carotenoids by a series of dehydrogenation reactions. In the photosynthetic bacterium Rhodopseudomonas capsulata mutations that affect carotenoid biosynthesis before colored carotenoids are formed have a "blue-green" phenotype as opposed to the "red" of wild type cells. We have extracted carotenoids from several blue-green mutants and found that two strains (BPY69 and BPY102) accumulate phytoene and no colored carotenoids. These mutants failed to dehydrogenate phytoene in an in vitro assay. However, dehydrogenation of this compound can be achieved in vitro by adding a cell-free extract from another blue-green mutant blocked earlier in the pathway. Genetic complementation and deletion mapping indicate that the gene crtI is responsible for the conversion of phytoene into colored carotenoids in these mutants.

Highlights

Carotenoids are membrane pigments present in all The main carotenoids accumulated by R. capsulata under photosynthetic organisms, providing essential photo- anaerobic conditions are hydroxyspheroidene andspheprotectivefunctions.The first carotenoidformedin roidene; under aerobic conditions spheroidenone predomithe pathway is phytoene, a colorlesscompound which nates (2)
In thephotosynthetic affecting the synthesis of colored carotenoids have been bacterium Rhodopseudomonas capsulata mutations mapped to a single genetic region of this bacterium (2, 4, 5)
In a previous study of the biosynthesis of colored carotenoids in the photosyntheticbacterium Rhodopseudomonas capsulata (2) (R. capsulatus in the nomenclature of Imhoff et al (3)) we showed that, as in other photosynthetic prokaryotes, there is an excellent correlation between the genetic and the biochemical information obtained from mutants affecting carotenoid biosynthesis

Summary

Introduction

Carotenoids are membrane pigments present in all The main carotenoids accumulated by R. capsulata under photosynthetic organisms, providing essential photo- anaerobic conditions are hydroxyspheroidene andspheprotectivefunctions.The first carotenoidformedin roidene; under aerobic conditions spheroidenone predomithe pathway is phytoene, a colorlesscompound which nates (2) (see Fig. 5). That affect carotenoid biosynthesis before coloredca- Mutations in four loci within this region Dehydrogenationofthis compound can be achieved in vitro by adding a cell-free extract from another blue-green mutant blockedearase.

Results

Conclusion