Photoregulation of the Carotenoid Biosynthetic Pathway in Albino and White Collar Mutants of Neurospora crassa

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Abstract
The conversion of isopentenyl pyrophosphate to phytoene in N. crassa requires both a soluble and a particulate fraction. Soluble and particulate enzyme fractions obtained from light-treated and dark-grown wild type, albino-1, albino-2, albino-3 and white collar-1 strains were mixed in various combinations, and the activity for conversion of [1-14C]isopentenyl pyrophosphate to phytoene was assayed. Apparently, albino-3 is defective in the soluble fraction; albino-2 is defective in the particulate fraction; the in vivo light treatment increases the enzyme activity in the particulate fraction; this light effect occurs in wild type, albino-1 and albino-3 strains; and enzyme activity is present in the particulate fraction obtained from the white collar-1 mutant, but the in vivo light treatment does not cause an increase in this activity. To measure directly the level of particulate enzyme activity, [14C]geranylgeranyl pyrophosphate [synthesized enzymically using extracts of pea cotyledons] was used as a substrate. Particulate enzyme fractions obtained from wild type, albino-1, and albino-3 strains incorporate [14C]geranylgeranyl pyrophosphate into phytoene, and this activity was higher in extracts obtained from light-treated cultures. The particulate fraction obtained from the white collar-1 mutant also incorporated [14C]geranylgeranyl pyrophosphate into phytoene, but the in vivo light treatment did not cause an increase in this activity. No incorporation occurred when particulate fractions obtained from either dark-grown or light-treated albino-2 cultures were assayed. The soluble enzyme fraction obtained from the albio-3 mutant was almost totally defective in enzyme activity required for the biosynthesis of [14C]geranylgeranyl pyrophosphate from [1-14C]isopentenyl pyrophosphate. An in vivo light treatment increases the level of this activity in wild type, albino-1, albino-2 and albino-3 strains, but not in the white collar-1 mutant. A model is presented to account for all results. Evidently the white collar-1 strain is a regulatory mutant blocked in the light induction process, whereas the albino-1, albino-2 and albino-3 strains are each defective for a different enzyme in the carotenoid biosynthetic pathway.