It is widely assumed that degradation of mitotic cyclins causes a decrease in mitotic cdc2/CDC28 kinase activity and thereby triggers the metaphase to anaphase transition. Two observations made on the budding yeast Saccharomcyces cerevisiae are inconsistent with this scenario: (i) anaphase occurs in the presence of high levels of kinase in cdc15 mutants and (ii) overproduction of a B-type mitotic cyclin causes arrest not in metaphase as previously reported but in telophase. Kinase destruction is therefore implicated in the exit from mitosis rather than the entry into anaphase. The behaviour of espi mutants shows in addition that kinase destruction can occur in the absence of anaphase completion. The execution of anaphase and the destruction of CDC28 kinase activity therefore appear to take place independently of one another.

Abstract. It is crucial to the eucaryotic cell cycle that the centrosome undergo precise duplication to generate the two poles of the mitotic spindle . In the budding yeast Saccharomyces cerevisiae, centrosomal functions are provided by the spindle pole body (SPB), which is duplicated at the time of bud emergence in GI of the cell cycle. Genetic control of this process has previously been revealed by the characterization of mutants in CDC31 and KARL, which prevent SPB duplication and lead to formation of a monopolar spindle . Newly isolated mutations described here (mpsl and mps2, for monopolar spindle) similarly

Allopolyploid hybridization serves as a major pathway for plant evolution, but in its early stages it is associated with phenotypic and genomic instabilities that are poorly understood. We have investigated allopolyploidization between Arabidopsis thaliana (2 n ‫؍‬ 2 x ‫؍‬ 10; n , gametic chromosome number; x , haploid chromosome number) and Cardaminopsis arenosa (2 n ‫؍‬ 4 x ‫؍‬ 32). The variable phenotype of the allotetraploids could not be explained by cytological abnormalities. However, we found suppression of 20 of the 700 genes examined by amplified fragment length polymorphism of cDNA. Independent reverse transcription-polymerase chain reaction analyses of 10 of these 20 genes confirmed silencing in three of them, suggesting that ‫ف‬ 0.4% of the genes in the allotetraploids are silenced. These three silenced genes were characterized. One, called K7, is repeated and similar to transposons. Another is RAP2.1 , a member of the large APETALA2 (AP2) gene family, and has a repeated element upstream of its 5 Ј end. The last, L6, is an unknown gene close to ALCOHOL DEHYDROGENASE on chromosome 1. CNG DNA methylation of K7 was less in the allotetraploids than in the parents, and the element varied in copy number. That K7 could be reactivated suggests epigenetic regulation. L6 was methylated in the C. arenosa genome. The present evidence that gene silencing accompanies allopolyploidization opens new avenues to this area of research.

In Saccharomyces cerevisiae, a highly ordered ring of 10-nm filaments is intimately associated with the plasma membrane within the neck of the bud. The ring is formed during early bud emergence and disappears when cytokinesis begins.

The interdependence of spindle plaque with other aspects of cell division and conjugation in Saccharomyces cerevisiae has been investigated. Three forms of the spindle plaque appear sequentially before the formation of the complete, intranuclear spindle. The single plaque is present initially in the mitotic cycle; it becomes transformed into a satellite-bearing single plaque during the latter part of G1. Subsequently, plaque duplication yields the double plaque characteristic of the early phase of budding, which coincides with the period of chromosome replication (S). The eventual separation of these plaques to form a complete spindle, with a single plaque at each pole, is nearly coincident with the completion of S. The form of the plaque differs in two independent cases of G1 arrest: the single plaque is found in a cell in stationary arrest of growth, whereas a cell arrested by mating factors in preparation for conjugation contains a satellite-bearing single plaque. The latter form is retained during zygote formation, where it serves as the initial site of fusion of each prezygotic nuceus with the other. This fusion results in the formation of a single zygotic nucleus with a satellite-bearing single plaque, which is subsequently transformed into a double plaque as the zygote buds. The double plaque is situated adjacent to the site of bud emergence in both vegetative cells and zygotes.

The previously described CLB1 and CLB2 genes encode a closely related pair of B-type cyclins. Here we present the sequences of another related pair of B-type cyclin genes, which we term CLB3 and CLB4. Although CLB1 and CLB2 mRNAs rise in abundance at the time of nuclear division, CLB3 and CLB4 are turned on earlier, rising early in S phase and declining near the end of nuclear division. When all possible single and multiple deletion mutants were constructed, some multiple mutations were lethal, whereas all single mutants were viable. All lethal combinations included the clb2 deletion, whereas the clbl clb3 clb4 triple mutant was viable, suggesting a key role for CLB2.

Mutants in the gene CDC34 of the yeast Saccharomyces cerevisiae are defective in the transition from G1 to the S phase of the cell cycle. This gene was cloned and shown to encode a 295-residue protein that has substantial sequence similarity to the product of the yeast RAD6 gene. The RAD6 gene is required for a variety of cellular functions including DNA repair and was recently shown to encode a ubiquitin-conjugating enzyme. When produced in Escherichia coli, the CDC34 gene product catalyzed the covalent attachment of ubiquitin to histones H2A and H2B in vitro, demonstrating that the CDC34 protein is another distinct member of the family of ubiquitin-conjugating enzymes. The cell cycle function of CDC34 is thus likely to be mediated by the ubiquitin-conjugating activity of its product.

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