Abstract— In addition to the direct inhibitory effects of visible light (cool‐white fluorescent, < 3500 lux) on growth and amino acid transport previously reported for cultures of the yeast Saccharomyces cereui‐siae (strain Y185 rho+) grown at 12°C in medium containing glucose, yeast carbon base, KH2PO4, ammonium ion and proline, we have found evidence for endogenous, light‐entrainable, self‐sustaining circadian and ultradian oscillators underlying both cell division and transport activity. Diurnal light (⋍ 3000 lux) cycles (LD), imposed on cultures previously grown in the dark, phased or synchronized cell number increase to a 24‐h period with bud release being confined primarily to the dark intervals (although not necessarily every cell divided during any given division ‘burst’). The observed division or budding rhythm freeran with a circadian period (⋍26h) only approximating 24 h for a number of days in constant darkness (DD) following prior entrainment by LD, thereby implicating an endogenous circadian clock. Further, a similar light‐entrainable circadian rhythm in the uptake of “C‐histidine or 14C‐lysine occurred in nondividing (or very slowly dividing) cultures during the “stationary” (infradian) phase of growth synchronized by a 24‐h LD cycle and then released into DD for as long as 10 days. Some experiments revealed a bimodal (ultradian) periodicity in both LD and DD with secondary peaks or shoulders occurring at intervals of ⋍12h, corresponding approximately to subjective ‘dawn’ and ‘dusk’. Transport in cultures of the Y185 rho‐ petite mutant, which lacks cytochromes a/a3, b and cl, could not be synchronized by LD cycles, a finding that is consistent with the hypothesis that these chromophores may be primary photoreceptors for synchronization of rhythms in this microorganism. Copyright © 1979, Wiley Blackwell. All rights reserved
