The characteristic orange color of N. crassa conidia is due to carotenoid pigments produced from mevalonic acid by a series of reactions that involve dehydrogenation, cyclization and cis-trans isomerization (Figure 9.8). Three mutations are known which block different steps in carotenoid biosynthesis. Any of these three mutations block the production of pigment, resulting in an albino (white) phenotype. The three genes were named al-1, al-2 and al-3 in the order of their discovery and all three genes have been cloned and sequenced.

In an attempt to overexpress carotenoid, Romano and Macino (1992) transformed a wild, orange colored strain with the wild al-1 + gene. Unexpectedly, the color in transformants was extinguished or suppressed—the transformants were white or pale yellow. Similarly, the duplication of chalcone synthase gene (chs ) involved in anthocyanin in flower petals results in the loss of flower color in plants where this phenomenon is known variously as co-suppression, repeat-induced gene suppression or homology-dependent

Heterokaryotic hph transformants


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