Rad52 recruitment is DNA replication independent and regulated by Cdc28 and the Mec1 kinase

Recruitment of the homologous recombination machinery to sites of double‐strand breaks is a cell cycle‐regulated event requiring entry into S phase and CDK1 activity. Here, we demonstrate that the central recombination protein, Rad52, forms foci independent of DNA replication, and its recruitment requires B‐type cyclin/CDK1 activity. Induction of the intra‐S‐phase checkpoint by hydroxyurea (HU) inhibits Rad52 focus formation in response to ionizing radiation. This inhibition is dependent upon Mec1/Tel1 kinase activity, as HU‐treated cells form Rad52 foci in the presence of the PI3 kinase inhibitor caffeine. These Rad52 foci colocalize with foci formed by the replication clamp PCNA. These results indicate that Mec1 activity inhibits the recruitment of Rad52 to both sites of DNA damage and stalled replication forks during the intra‐S‐phase checkpoint. We propose that B‐type cyclins promote the recruitment of Rad52 to sites of DNA damage, whereas Mec1 inhibits spurious recombination at stalled replication forks.     

Gene assignment by means of somatic cell hybrids: a new method for the analysis of data

Summary A statistical approach to the interpretation of data from gene assignment with somatic cell hybrids is presented. The observed data are analysed under a variety of hypotheses. The fit to the hypotheses is compared by means of the likelihood obtained under a given hypothesis. Two of these hypotheses are related to fundamental questions: is a gene responsible for the enzyme observation and if so, is that gene located on a specific chromosome or could it change its position and be sometimes on chromosome j and, in another hybrid line, on chromosome k? The other hypotheses concern the assignment of the gene to just one of the chromosomes.To improve the traditional data analysis approach we considered additional information: the uncertainties and possible errors of laboratory methods in all our calculations and the length of the donor chromosomes in connection with one specific hypothesis.This method allows us to account for the reliability of the investigation methods and the nature of the hybrid lines involved. Data can be evaluated at different error probabilities within a realistic range in order to compare and discuss results.     

纯系间数量性状主基因差异的遗传分析

将Ｅｌｓｔｏｎ模型应用于存在主基因差异的系间杂交的分离世代分析,提出利用似然函数分析数量性状的尺度效应、主基因分离比例以及主基因效应和微基因效应的方法,并应用于水稻遗传实验,结果表明,半矮秆籼稻品种南京１１号带有的隐性矮秆主基因,效应大约为４０－５６ｃｍ,微基因效应以加性为主,且主、微型基因存在显著互作,但互作和微基因效应均远小于基因效应。     

Evolution of the Mitochondrial Cytochrome Oxidase II Gene in Collembola

The sequence of the mitochondrial COII gene has been widely used to estimate phylogenetic relationships at different taxomonic levels across insects. We investigated the molecular evolution of the COII gene and its usefulness for reconstructing phylogenetic relationships within and among four collembolan families. The collembolan COII gene showed the lowest A + T content of all insects so far examined, confirming that the well-known A + T bias in insect mitochondrial genes tends to increase from the basal to apical orders. Fifty-seven percent of all nucleotide positions were variable and most of the third codon positions appeared free to vary. Values of genetic distance between congeneric species and between families were remarkably high; in some cases the latter were higher than divergence values between other orders of insects. The remarkably high divergence levels observed here provide evidence that collembolan taxa are quite old; divergence levels among collembolan families equaled or exceeded divergences among pterygote insect orders. Once the saturated third-codon positions (which violated stationarity of base frequencies) were removed, the COII sequences contained phylogenetic information, but the extent of that information was overestimated by parsimony methods relative to likelihood methods. In the phylogenetic analysis, consistent statistical support was obtained for the monophyly of all four genera examined, but relationships among genera/families were not well supported. Within the genus Orchesella, relationships were well resolved and agreed with allozyme data. Within the genus Isotomurus, although three pairs of populations were consistently identified, these appeared to have arisen in a burst of evolution from an earlier ancestor. Isotomurus italicus always appeared as basal and I. palustris appeared to harbor a cryptic species, corroborating allozyme data. Received: 12 January 1996 / Accepted: 10 August 1996