Structural studies on bacterial carotenoids and their biosynthetic implications.

Detailed studies of the properties of carotenoids isolated from diphenylamine-inhibited cultures of Rhodospirillum rubrum have revealed a number of novel structures which indicate new features of carotenoid biosynthesis in the photosynthetic bacteria. The dehydrogenation of phytoene to coloured carotenoids occurs by a sequence which is different from that in higher plants in that 7,8,11,12-tetrahydrolycopene, rather than c-carotene, is the immediate precursor of neurosporene. Both neurosporene and 7,8,11,12tetrahydrolycopene undergo hydration, methylation and dehydrogenation to yield spheroidene and 11', 12'-dihydrospheroidene respectively; all the intermediates in these pathways have been identified. These pathways represent alternative routes of anhydrorhodovibrin and spirilloxanthin biosynthesis. INTRODUCTION The carotenoid pigments, like other natural isoprenoid compounds, are formed from mevalonic acid through isopentenyl pyrophosphate. The condensation of isopentenyl pyrophosphate and dimethylallyl pyrophosphate leads to the formation of geranyl pyrophosphate and this, in turn, is converted into farnesyl pyrophosphate and then into geranylgeranyl pyrophosphate. The isoprenoid chains of two C20-pyrophosphate units condense, tail-to-tail, to form the first C40-carotenoid precursor, phytoene, which is then dehydrogenated to form the coloured carotenoids. PATHWAYS OF PHYTOENE DEHYDROGENATION The only pathway of coloured carotenoid formation that has been demonstrated, and which is generally assumed to operate in all carotenogenic organisms, is the sequence of reactions first proposed by Porter and Lincoln'6 on the basis of their observations on the carotenoids of tomato fruit. Any uncertainty of the structures of the intermediates in this sequence were eliminated by the unambiguous synthesis of phytoene [1], phytofluene [2], c-carotene [3] and neurosporene [4] and the comparison of the synthetic compounds with natural samples isolated from carrot oil4'5. The structures of these intermediates indicate that phytoene undergoes a series of stepwise dehydrogenations through phytofluene, l-carotene and neurosporene to 545