Evidence on intracellular sites of terminal synthesis of carotenoids in Mucor hiemalis Wehm.

Introduction Available evidence on carotenoid biosynthesis indicates that a general pathway is utilized in all carotenogenic systems, at least in the primary stages (Jensen 1965). The sequential dehydrogenation of the colorless Co precurs~rs to the colored carotenes has been extensively studied in the photosynthe ... t":'"ic----........bacteria (Stanier 1959) and appears to follow the Porter-Lincoln sequence, as developed by Porter and Anderson (1962). The terminal stages of biosynthesis and a proposed cyclization mechanism in the carrot, however, appear to differ from that sequence in the condensation of tJ-carotene directly from an acyclic precursor (Goodwin and Williams 1965). Useful data have been obtained from a cell-free system of Phycomyces blakesleeanus (Yokoyama et al. 1962) through the study of single steps in synthesis, but it is desirable to develop and extend the technique to other organisms. Precise knowledge of the site of carotenoid synthesis would permit effective concentration of a cell-free system. The problem of post-synthetic accumulation of carotenoids is unresolved. The discrepancy between results of diphenylamine experiments on Rhodospirillum rubrum (Stanier 1959) and Phycomyces blakesleeanus (Goodwin et al. 1953; Varma et al. 1959) has been explained on the basis of the availability of carotenoids to further enzymic transformation (Stanier 1959). This explanation suffers from the facts that the yellow droplets observed in mycelium of P. blakesleeanus have never been demonstrated to be lipids with dissolved carotenes, and that the synthesizing enzyme system has not yet been located. The following experiments attempt to do so.