Insights into the identification and evolutionary conservation of key genes in the transcriptional circuits of meiosis initiation and commitment in budding yeast.

Initiation of meiosis in budding yeast does not commit the cells for meiosis. Thus, two distinct signaling cascades may differentially regulate meiosis initiation and commitment in budding yeast. To distinguish between the role of these signaling cascades, we reconstructed protein-protein interaction networks and gene regulatory networks with upregulated genes in meiosis initiation and commitment. Analyzing the integrated networks, we identified four master regulators (MRs) [Ume6p, Msn2p, Met31p, Ino2p], three transcription factors (TFs), and 279 target genes (TGs) unique for meiosis initiation, and three MRs [Ndt80p, Aro80p, Rds2p], 11 TFs, and 948 TGs unique for meiosis commitment. Functional enrichment analysis of these distinct members from the transcriptional cascades for meiosis initiation and commitment revealed that nutritional cues rewire gene expression for initiating meiosis and chromosomal recombination commits cells to meiosis. As meiotic chromosomal recombination is highly conserved in eukaryotes, we compared the evolutionary rate of unique members in the transcriptional cascade of two meiotic phases of Saccharomyces cerevisiae with members of the phylum Ascomycota, revealing that the transcriptional cascade governing chromosomal recombination during meiosis commitment has experienced greater purifying selection pressure (P value = 0.0013, 0.0382, 0.0448, 0.0369, 0.02967, 0.04937, 0.03046, 0.03357 and < 0.00001 for Ashbya gossypii, Yarrowia lipolytica, Debaryomyces hansenii, Aspergillus fumigatus, Neurospora crassa, Kluyveromyces lactis, Schizosaccharomyces pombe, Schizosaccharomyces cryophilus, and Schizosaccharomyces octosporus, respectively). This study demarcates crucial players driving meiosis initiation and commitment and demonstrates their differential rate of evolution in budding yeast.

An evolutionary and functional assessment of regulatory network motifs.

BACKGROUND: Cellular functions are regulated by complex webs of interactions that might be schematically represented as networks. Two major examples are transcriptional regulatory networks, describing the interactions among transcription factors and their targets, and protein-protein interaction networks. Some patterns, dubbed motifs, have been found to be statistically over-represented when biological networks are compared to randomized versions thereof. Their function in vitro has been analyzed both experimentally and theoretically, but their functional role in vivo, that is, within the full network, and the resulting evolutionary pressures remain largely to be examined. RESULTS: We investigated an integrated network of the yeast Saccharomyces cerevisiae comprising transcriptional and protein-protein interaction data. A comparative analysis was performed with respect to Candida glabrata, Kluyveromyces lactis, Debaryomyces hansenii and Yarrowia lipolytica, which belong to the same class of hemiascomycetes as S. cerevisiae but span a broad evolutionary range. Phylogenetic profiles of genes within different forms of the motifs show that they are not subject to any particular evolutionary pressure to preserve the corresponding interaction patterns. The functional role in vivo of the motifs was examined for those instances where enough biological information is available. In each case, the regulatory processes for the biological function under consideration were found to hinge on post-transcriptional regulatory mechanisms, rather than on the transcriptional regulation by network motifs. CONCLUSION: The overabundance of the network motifs does not have any immediate functional or evolutionary counterpart. A likely reason is that motifs within the networks are not isolated, that is, they strongly aggregate and have important edge and/or node sharing with the rest of the network.

The genetic epidemiology of leprosy in a Brazilian population.

Data on leprosy patients have been obtained from the Dispensary of Leprosy of Campinas, São Paulo, where records on practically all cases of leprosy in the Campinas area during the period 1960-70 are filed. The whole sample comprises 10,886 individuals, distributed among 1,568 families. Complex segregation analysis was utilized to determine the nature of the genetic factors that may operate on leprosy and its subtypes. The results suggest the presence of a recessive major gene controlling susceptibility to leprosy per se, with frequency of approximately .05, although there are deviations from the expected Mendelian segregation proportions. Possible etiologic heterogeneity was examined by considering two subtypes separately: for lepromatous leprosy and tuberculoid leprosy there are suggestions for a segregating major effect; however, Mendelian transmission could not be demonstrated in either case. Therefore, there is no evidence to suggest unique genetic determinants for leprosy subtypes.

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.

HLA-linked control of susceptibility to tuberculoid leprosy and association with HLA-DR types.

In an attempt to confirm an HLA-linked effect on the course of Mycobacterium leprae infection observed in families from Surinam (South America), we conducted a similar family study in an endemic area in India. We observed a significant (P less than .05) excess of identical HLA-GLO haplotypes only from healthy parents among siblings affected with tuberculoid leprosy. Compared with healthy controls, unrelated patients with tuberculoid leprosy (n = 15) showed a significant heterogeneity at the HLA-DR locus (P less than .05). This heterogeneity was caused by an increased frequency of HLA-DRw2 (.93 versus .53, P less than .05), particularly of DRw2 homozygotes (.53 versus .11, P less than .005), and a decreased frequency of HLA-DRw6 (.07 versus .58, P less than .005). We observed a significant (P = .03) preferential segregation of DRw2 from DRw2 heterozygous parents not affected with tuberculoid leprosy to children with the tuberculoid type of the disease. These data confirm an HLA-linked control of susceptibility to tuberculoid leprosy only, and suggest a recessive inheritance of this trait for which HLA-Drw2 appears to be a genetic marker.

Electron microscopy of ascus formation in the yeast debaryomyces hansenii.

Ascus formation in Debaryomyces hansenii includes fusion of two cells, usually mother and daughter while still attached to each other, through short protuberances developed from the cross wall between them. Nuclear fusion takes place in the channel connecting the two cells; meiosis apparently occurs in the mother cell. Generally, only one lobe of the meiotic nucleus is surrounded by a prospore wall and it becomes the nucleus of a spore with a warty wall. The rest of the nucleus disappears. The spores germinate by swelling in the ascus and forming one or more buds.

Spindles, spindle plaques, and meiosis in the yeast Saccharomyces cerevisiae (Hansen).

The intranuclear spindle of yeast has an electron-opaque body at each pole. These spindle plaques lie on the nuclear envelope. During mitosis the spindle elongates while the nuclear membranes remain intact. After equatorial constriction there are two daughted nuclei, each with one spindle plaque. The spindle plaque then duplicates so that two side-by-side plaques are produced. These move rapidly apart and rotate so that they bracket a stable 0.8 microm spindle. Later, during mitosis, this spindle elongates, etc. Yeast cells placed on sporulation medium soon enter meiosis. After 4 hr the spindle plaques of the more mature cells duplicate, producing a stable side-by-side arrangement. Subsequently the plaques move apart to bracket a 0.8 microm spindle which immediately starts to elongate. When this meiosis I spindle reaches its maximum length of 3-5 microm, each of the plaques at the poles of the spindle duplicates and the resulting side-by-side plaques increase in size. The nucleus does not divide. The large side-by-side plaques separate and bracket a short spindle of about 1 microm which elongates gradually to 2 or 3 microm. Thus there are two spindles within one nucleus at meiosis II. To the side of each of the four plaques a bulge forms on the nucleus. The four bulges enlarge while the original nucleus shrinks. These four developing ascospore nuclei are partially surrounded by cytoplasm and by a prospore wall which originates from the cytoplasmic side of the spindle plaque. Eventually the spore nuclei pinch off and the spore wall closes. In some of the larger yeast cells this development is completed after 8 hr on sporulation medium.