Commitment to the mitotic cell cycle in yeast in relation to meiosis

Under restrictive vegetative conditions, cells of cell-division cycle (cdc) temperature-sensitive mutants arrest at specific points in the cycle. Meiotic and mitotic behaviour of such arrested cells was examined under permissive sporulation conditions. Those mutants which were committed to mitosis at their specific point of arrest finished the cell cycle and could only then go into meiosis. It was found that commitment to mitosis occurred early in the cell cycle, prior to DNA replication, and that this commitment was dependent upon the gene function of cdc4.     

Uridine uptake by nerve cells of the grivet monkey and its relation to cytoplasmic ribonucleic acid concentration

Following intravenous injection of tritiated uridine to a grivet monkey, the uptake in nerve cell nuclei was determined autoradiographically in different brain regions, in the choroid plexus, liver and kidney. It is shown (1) that there are obvious differences in the labelling in different brain areas, and (2) that the labelling, compared with earlier results in mice, is the same in some regions but different in others.The uridine labelling was inversely related to the microspectrophotometrically determined cytoplasmic concentration of ribonucleic acid. This might indicate that cells with a high uridine count and a low cytoplasmic ribonucleic acid concentration have a high metabolic activity.     

Translocation of zinc from vacuole to nucleus during yeast meiosis

A novel staining procedure employing the UV fluorochrome DAPI (4',6-diamidino-2-phenylindole X 2HCl) and dithizone (diphenylthiocarbazone) was developed for microcytochemical determination of sites of zinc localization in Saccharomyces cerevisiae Hansen. In vegetative cells vacuolar polyphosphate bodies stained with dithizone, whereas in sporulating cells nucleoli and centriolar plaques were dithizone-positive. Hence, dithizone not only permitted localization of zinc but also indicated zinc translocation from vacuolar to nuclear compartments during differentiation from the vegetative to sporulated state.     

Morphogenesis of the synapton during yeast meiosis

The formation of the synapton (synaptonemal complex) was followed by an electron microscopic examination of large samples of Saccharomyces cerevisiae cells at various stages of meiosis. Three temperature-sensitive mutants were used, cdc4, cdc5 and cdc7, which undergo a slow but normal meiosis at 25° C. At the restrictive temperature of 34° C, cdc4 and cdc5 arrest at an advanced enough stage of meiosis to allow the study of synapton morphogenesis. Based on the frequencies of nuclear structures, we describe the formation of the central region and central elements of the synapton in the dense body, which may be part of the nucleolus. This process occurs during early meiotic stages, concomittantly with recombination commitment and premeiotic DNA replication. Mature synaptons usually appear after premeiotic S, at the pachytene stage, and later disappear. A possible intermediate stage in this disappearance is found in arrested cdc5 cells, which contain paired lateral elements without central elements. — Following the frequencies of spindle plaque configurations, we conclude that the plaques in meiosis duplicate once at the beginning of the main DNA replication, as is also observed prior to mitosis. In contrast to mitotic cells, however, meiotic plaques remain duplicated for a long period, until the synaptons disappear, and only then separate from each other to form a spindle. During late stages of the first meiotic division, the outer plates of the spindle plaques thicken, to duplicate later and give the second division spindles. The characteristically thick outer plate may have a role in the formations of the ascospore wall.     

Commitment to meiosis in fission yeast

Summary Mutants of Schizosaccharomyces pombe blocked during meiosis were analysed with respect to the induction of diploid mitotic division. Wild type zygotes of this yeast can form diploid colonies with a low probability (ca. 1%) when they are transferred to fresh growth medium. Mutants of three genes affecting meiosis responded to the shift by forming diploid colonies with high yield (ca. 80% of the zygotes). Hence, commitment to meiosis could not be reached by such zygotes. Zygotes of a fourth meiosis-I-deficient mutant were unable to yield diploid colonies more effectively than wild type zygotes. Morphological changes were prominent in the mutant zygotes not reaching commitment (as well as in mutant cells that could not complete conjugation), indicating that activities needed during early stages of conjugation were not turned off in these zygotes.This work was supported by NIH grant GM 13234 initially, and by DFG (SFB 46).