In Flavobacterium R1519, nicotine blocks zeaxanthin biosynthesis by specifically inhibiting the cyclization reaction. Lycopene (at high nicotine concentrations, e.g. 7.5mm) and rubixanthin (at low nicotine concentration, e.g. 1mm) replace zeaxanthin as the main carotenoid. On removal of the nicotine lycopene is converted into β-carotene under anaerobic conditions and into zeaxanthin in the presence of O2. The conversion in vivo of β-carotene into zeaxanthin was also demonstrated. Cyclization (an anaerobic process) thus precedes hydroxylation (O2-requiring) in the biosynthesis of zeaxanthin. The conversion in vivo of rubixanthin into β-cryptoxanthin and into zeaxanthin was demonstrated, thus indicating the operation of alternative pathways of zeaxanthin biosynthesis. Several alternative biosynthetic pathways are considered and the results are also discussed in terms of reaction sequences of carotenoid ‘half-molecules’.
