Recent Studies of Carotenoid Biosynthesis

The effects of nicotine on carotenoid biosynthesis have been investigated in a non-photosynthetic Flavobacterium strain 0147, and in two photosynthetic bacteria, Rhodopseudomonas spheroides and Rhodomicrobium vanrtielii. In Flavobacterium, nicotine inhibits the synthesis of the nOrmal main carotenoid, zeaxanthin, and causes an accumulation of lycopene at high nicotine concentration, and rubixanthin at lower nicotine concentration. On removal of the inhibitor, the accumulated lycopene is converted under anaerobic conditions into 13-carotene, which in turn is converted into zeaxanthin in the presence of oxygen. Similar in vivo conversions of rubixanthin into 3-cryptoxanthin (anaerobic) and zeaxanthin (aerobic) have been demonstrated. Two alternative routes from lycopene to zeaxanthin are therefore possible in Flavobacterium, via 13-carotene or via rubixanthin. Nicotine also inhibits production of the normal carotenoids in two photosynthetic bacteria. In Rhodopseudomonas spheroides, at high nicotine concentrations, the normal main carotenoid spheroidene is replaced by neurosporene, hut at low nicotine concentrations the amount of spheroidene is increased, while the hydroxyspheroidene content decreases to zero. This offers support for the postulated pathway neurosporene —* spheroidene —+ hydroxyspheroidene. In Rhodomicrobium vannielii, formation of the cyclic J3-carotene is completely inhibited at low nicotine concentrations, and at higher nicotine concentrations the normal main carotenoid, rhodopin, is greatly decreased, while lycopene becomes the main product. In addition to inhibiting the cyclization reaction, nicotine thus apparently inhibits the hydration of the terminal (C-I) double bond in the biosynthesis of the characteristic hydroxyand methoxycarotenoids of photosynthetic bacteria. In Rhodopseudomonas spheroides, neurosporene appears to be the key intermediate in the formation of other carotenoids, whereas lycopene plays the most important role in Rhodomicrobium vannielii. Present address: Department of Botany, University of Liverpool, Liverpool L69 3BX. Present address: Volcani Institute of Agricultural Research, National and University Institute of Agriculture. Bet Dagan, Israel