The nature of the inorganic carbon (C,) species actively taken up by cyanobacteria CO2 or HC03-has been investigated. The kinetics of CO2 uptake, as well as that of HC03-uptake, indicated the involvement of a saturable process. The apparent affinity of the uptake mechanism for CO2 was higher than that for HCO3-. Though the calculated V. was the same in both cases, the maximum rate of uptake actually observed was higher when HC03-was supplied. C. uptake was far more sensitive to the carbonic anhydrase inhibitor ethoxyzolamide when CO2 was the species supplied. Observations of photosynthetic rate as a function of intracellular C. level (following supply of CO2 or HCO3-for 5 seconds) led to the inference that HC03-is the species which arrives at the inner membrane surface, regardless of the species supplied. When the two species were supplied simultaneously, mutual inhibition of uptake was observed.On the basis of these and other results, a model is proposed postulating that a carboic anhydrase-like subunit of the C. transport apparatus binds CO2 and releases HC03-at or near a membrane porter. The latter transports HC03-ions to the cell interior.The observed capability of green algae and cyanobacteria to accumulate Ci2 within the cells has been attributed to the operation of a mechanism for active transport of HCO3 ions (for review, see Lucas [8]). However, the observation that supply of '4C02 to green algae (5, 9, 1 1) and cyanobacteria (2) leads to faster accumulation of C. and acid-stable '4C than does HCO3 supply has prompted the proposal that CO2 may in fact be the species actively transported. In the present study we have investigated the uptake of each C, species within a brief period of its supply to Anabaena variabilis cells and, further, have examined interactions between CO2 and HCO3 during the uptake process. Data obtained bear on the molecular mechanism involved in C. uptake in cyanobacteria. MATERIALS AND METHODSCultures of Anabaena variabilis M-3 (from the collection of Tokyo University) were grown as described earlier (6). Accumulation of acid stable (photosynthetic products) and labile (inorganic) carbon were determined using the filtering centrifugation technique (6, 7 injected into closed microfuge tubes containing 100 mm Mes (pH 2.4). Following this injection the pH rose to 5.1 (maximum). The solution in the tube was mixed for 60 s with a syringe and the required volume withdrawn and injected into the cell suspension. Another sample was injected into 0.1 N NaOH. The radioactivity in the letter sample was assessed in order to correct for the loss of 14C02 from the microfuge tube. This procedure enabled accurate determination of the concentration of CO2 supplied, and ensured that the specific activity of the C. supplied was the same, whether presented as CO2 or HCO3 . The injection of aliquots of the H'4CO3 or '4CO2 stocks changed the pH of the cell suspension (in 50 mM Hepes-NaOH, pH 8.0) to only a minor degree (maximum 0.1 pH units). This change was taken into account whenever the actual...

In the mitotic cell cycle of the yeast Saccharomyces cerevisiae, the sister chromatid is preferred over the homologous chromosome (non-sister chromatid) as a substrate for DNA double-strand break repair. However, no genes have yet been shown to be preferentially involved in sister chromatid-mediated repair. We developed a novel method to identify genes that are required for repair by the sister chromatid, using a haploid strain that can embark on meiosis. We show that the recombinational repair gene RAD54 is required primarily for sister chromatid-based repair, whereas TID1, a yeast RAD54 homologue, and the meiotic gene DMC1, are dispensable for this type of repair. Our observations suggest that the sister chromatid repair pathway, which involves RAD54, and the homologous chromosome repair pathway, which involves DMC1, can substitute for one another under some circumstances. Deletion of RAD54 in S.cerevisiae results in a phenotype similar to that found in mammalian cells, namely impaired DNA repair and reduced recombination during mitotic growth, with no apparent effect on meiosis. The principal role of RAD54 in sister chromatid-based repair may also be shared by mammalian and yeast cells.

We present a scheme for locating double‐strand breaks (DSBs) in meiotic chromosomes of Saccharomyces cerevisiae, based on the separation of large DNA molecules by pulsed field gel electrophoresis. Using a rad50S mutant, in which DSBs are not processed, we show that DSBs are widely induced in S. cerevisiae chromosomes during meiosis. Some of the DSBs accumulate at certain preferred sites. We present general profiles of DSBs in chromosomes III, V, VI and VII. A map of the 12 preferred sites on chromosome III is presented. At least some of these sites correlate with known ‘hot spots’ for meiotic recombination. The data are discussed in view of current models of meiotic recombination and chromosome segregation.

Background: Spore germination in the yeast Saccharomyces cerevisiae is a process in which nondividing haploid spores re-enter the mitotic cell cycle and resume vegetative growth. To study the signals and pathways underlying spore germination we examined the global changes in gene expression and followed cell-cycle and germination markers during this process.

Background: Meiosis in budding yeast is coupled to the process of sporulation, where the four haploid nuclei are packaged into a gamete. This differentiation process is characterized by a point of transition, termed commitment, when it becomes independent of the environment. Not much is known about the mechanisms underlying commitment, but it is often assumed that positive feedback loops stabilize the underlying gene-expression cascade.