Correlation between in vivo and in vitro carotenogenesis in Phycomyces

External stimuli and mutations in several car genes modify the β-carotene content of the fungus Phycomyces blakesleeanus. Cell-free extracts from unstimulated agar-grown wild-type mycelia incorporate radioactivity from sodium [2-₁₄C]mevalonate into carotenes and squalene, in relative proportion of ca 3: 100. Higher incorporations into carotenes are found in extracts from (i) mycelia grown in the light or in the presence of retinol or dimethyl phthalate, although these agents have no effect on extracts from unstimulated mycelia; (ii) intersexual heterokaryons, i.e., cells with constitutive sexual stimulation; (iii) carB and carR mutants, unable to synthesize β-carotene because of blocks in the pathway; (iv) carS, carD and carI mutants with various alterations in pathway regulation. The results fit previous hypotheses on the regulation of carotenogenesis in Phycomyces and suggest that activators of carotenogenesis work through an increased gene expression. The results depend on the culture conditions: wild-type mycelia grown in liquid culture are more carotenogenic, both in vivo and in vitro, than those grown on agar.