INTRODUCTION OF NEW CAROTENOIDS INTO THE BACTERIAL PHOTOSYNTHETIC APPARATUS BY COMBINING THE CAROTENOID BIOSYNTHETIC PATHWAYS OF ERWINIA-HERBICOLA AND RHODOBACTER-SPHAEROIDES

Carotenoids have two major functions in bacterial photosynthesis, phot oprotection and accessory light harvesting. The genes encoding many ca rotenoid biosynthetic pathways have now been mapped and cloned in seve ral different species, and the availability of cloned genes which enco de the biosynthesis of carotenoids not found in the photosynthetic gen us Rhodobacter opens up the possibility of introducing a wider range o f foreign carotenoids into the bacterial photosynthetic apparatus than would normally be available by producing mutants of the native biosyn thetic pathway. For example, the crt genes from Erwinia herbicola, a g ram-negative nonphotosynthetic bacterium which produces carotenoids in the sequence of phytoene, lycopene, beta-carotene, beta-cryptoxanthin , zeaxanthin, and zeaxanthin glucosides, are clustered within a 12.8-k b region and have been mapped and partially sequenced. In this paper, part of the E. herbicola crt cluster has been excised and expressed in various crt strains of Rhodobacter sphaeroides. This has produced lig ht-harvesting complexes with a novel carotenoid composition, in which the foreign carotenoids such as p-carotene function successfully in li ght harvesting. The outcome of the combination of the crt genes in R. sphaeroides with those from E. herbicola has, in some cases, resulted in an interesting rerouting of the expected biosynthetic sequence, whi ch has also provided insights into how the various enzymes of the caro tenoid biosynthetic pathway might interact. Clearly this approach has considerable potential for studies on the control and organization of carotenoid biosynthesis, as well as providing novel pigment-protein co mplexes for functional studies.