Transposable mating type genes in Saccharomyces cerevisiae.

A functional copy of the alpha mating type gene of Saccharomyces cerevisiae has been cloned by transformation in yeast. Using the Southern Blotting procedure it has been shown that three distinct genetic loci implicated in mating type interconversion (HML, HMR and MAT) contain sequences homologous to the clone fragment. The restriction fragment associated with each locus exhibits a characteristic size which can be correlated with the mating type allele present at that locus. The characteristic size difference between the a and alpha genetic elements made it possible to demonstrate that the homothallic interconversion of mating types in this yeast occurs by DNA rearrangement as proposed in the 'cassette hypothesis'.     

Homing of a DNA endonuclease gene by meiotic gene conversion in Saccharomyces cerevisiae.

An unusual protein splicing reaction joins the N-terminal segment (A) and the C-terminal segment (C) of the 119K primary translation product (ABC) of the yeast VMA1 gene to yield a 69K vacuolar H(+)-ATPase subunit (AC) and an internal 50K polypeptide (B). This 50K protein is a site-specific DNA endonuclease that shares 34% identity with the homothallic switching endonuclease. The site cleaved by the VMA1-derived endonuclease exists in a VMA1 allele that lacks the derived endonuclease segment of the open reading frame. Cleavage at this site only occurs during meiosis and initiates 'homing', a genetic event that converts a VMA1 allele lacking the endonuclease coding sequence into one that contains it.     

Asymmetric gene conversion at inserted segments on yeast mitochondrial DNA.

Different molecular weight forms of the protein product of the yeast mitochondrial gene var 1 are shown at arise by a process of asymmetric gene conversion. These different forms can be accounted by two DNA segments, 36 and 57 base pairs long, present in one allelic form of the var 1 structural gene, which can be inserted independently and at different frequencies into other var 1 alleles.     

High-resolution mapping of meiotic crossovers and non-crossovers in yeast

Meiotic recombination has a central role in the evolution of sexually reproducing organisms. The two recombination outcomes, crossover and non-crossover, increase genetic diversity, but have the potential to homogenize alleles by gene conversion. Whereas crossover rates vary considerably across the genome, non-crossovers and gene conversions have only been identified in a handful of loci. To examine recombination genome wide and at high spatial resolution, we generated maps of crossovers, crossover-associated gene conversion and non-crossover gene conversion using dense genetic marker data collected from all four products of fifty-six yeast (Saccharomyces cerevisiae) meioses. Our maps reveal differences in the distributions of crossovers and non-crossovers, showing more regions where either crossovers or non-crossovers are favoured than expected by chance. Furthermore, we detect evidence for interference between crossovers and non-crossovers, a phenomenon previously only known to occur between crossovers. Up to 1% of the genome of each meiotic product is subject to gene conversion in a single meiosis, with detectable bias towards GC nucleotides. To our knowledge the maps represent the first high-resolution, genome-wide characterization of the multiple outcomes of recombination in any organism. In addition, because non-crossover hotspots create holes of reduced linkage within haplotype blocks, our results stress the need to incorporate non-crossovers into genetic linkage analysis.     

Gene conversion between duplicated genetic elements in yeast

The mitotic recombination behaviour of a duplication of the his4 region on chromosome III in the yeast Saccharomyces cerevisiae was studied. The major recombination event between the duplicated segments is gene conversion unassociated with reciprocal recombination. The rad52-1 mutation preferentially decreases mitotic gene conversion. These results suggest that mitotic gene conversion may occur by a different pathway from that occurring in meiosis. This mitotic gene conversion may be important in yeast mating type interconversion and the maintenance of sequence homogeneity in families of repeated eukaryotic genes.     

Mismatch-specific post-meiotic segregation frequency in yeast suggests a heteroduplex recombination intermediate

Post-meiotic segregation of alleles, which is seen, for example, in the 5:3 distribution of alleles in the products of a single meiosis in fungi, has been thought to be due to the non-repair of heteroduplex regions formed during genetic recombination. In current models of genetic recombination, heteroduplex DNA is formed either as the primary intermediate generated by two interacting non-sister chromatids or as a short region flanking a double-stranded gap. The frequency of post-meiotic segregation differs for different alleles, and this is presumed to reflect the varying efficiencies with which different types of mismatches in the heteroduplex are repaired. To gain some insight into this process, we have now determined the nucleotide sequences of various yeast alleles with different post-meiotic segregation frequencies and compared the mismatches predicted to occur in heteroduplexes of these alleles with wild-type DNA with those repaired with varying efficiency in bacterial systems. A striking correlation is observed, with the mismatches predicted for high post-meiotic segregation frequency alleles being similar to mismatches repaired with low efficiency in bacteria. These results support the view that postmeiotic segregation frequency reflects heteroduplex repair efficiency and the contention that meiotic gene conversion is the result of the successful repair of heteroduplex mismatches.     

Genome-wide non-mendelian inheritance of extra-genomic information in Arabidopsis

A fundamental tenet of classical mendelian genetics is that allelic information is stably inherited from one generation to the next, resulting in predictable segregation patterns of differing alleles. Although several exceptions to this principle are known, all represent specialized cases that are mechanistically restricted to either a limited set of specific genes (for example mating type conversion in yeast) or specific types of alleles (for example alleles containing transposons or repeated sequences). Here we show that Arabidopsis plants homozygous for recessive mutant alleles of the organ fusion gene HOTHEAD (HTH) can inherit allele-specific DNA sequence information that was not present in the chromosomal genome of their parents but was present in previous generations. This previously undescribed process is shown to occur at all DNA sequence polymorphisms examined and therefore seems to be a general mechanism for extra-genomic inheritance of DNA sequence information. We postulate that these genetic restoration events are the result of a template-directed process that makes use of an ancestral RNA-sequence cache.     

The yeast STE6 gene encodes a homologue of the mammalian multidrug resistance P-glycoprotein

Mammalian tumours displaying multidrug resistance overexpress a plasma membrane protein (P-glycoprotein), which is encoded by the MDR1 gene and apparently functions as an energy-dependent drug efflux pump. Tissue-specific expression of MDR1 and other members of the MDR gene family has been observed in normal cells, suggesting a role for P-glycoproteins in secretion. We have isolated a gene from the yeast Saccharomyces cerevisiae that encodes a protein very similar to mammalian P-glycoproteins. Deletion of this gene resulted in sterility of MATa, but not of MAT alpha cells. Subsequent analysis revealed that the yeast P-glycoprotein is the product of the STE6 gene, a locus previously shown to be required in MATa cells for production of a-factor pheromone. Our findings suggest that the STE6 protein functions to export the hydrophobic a-factor lipopeptide in a manner analogous to the efflux of hydrophobic cytotoxic drugs catalysed by the related mammalian P-glycoprotein. Thus, the evolutionarily conserved family of MDR-like genes, including the hlyB gene of Escherichia coli and the STE6 gene of S. cerevisiae, encodes components of secretory pathways distinct from the classical, signal sequence-dependent protein translocation system.     

Microconversion between murine H-2 genes integrated into yeast

Patchwork homology observed between divergent members of polymorphic multigene families is thought to reflect evolution by short-tract gene conversion (nonreciprocal recombination), although this mechanism cannot usually be confirmed in higher organisms. In contrast to meiotic conversions observed in laboratory yeast strains, apparent conversions between polymorphic sequences, such as the class I loci of the major histocompatibility complex (MHC), are short and do not seem to be associated with reciprocal recombination (crossover, exchanges). We have now integrated two nonallelic murine class I genes into yeast to characterize their meiotic recombination. We found no crossovers between the MHC genes, but short-tract 'microconversions' of 1-215 base-pairs were observed in about 6% of all meioses. Strikingly, one of these events was accompanied by a single base-pair mutation. These results underscore both the importance of meiotic gene conversion and sequence heterology in determining conversion patterns between divergent genes.     

Intragenic recombination leads to pilus antigenic variation in Neisseria gonorrhoeae

The pilus of the bacterium Neisseria gonorrhoeae is a fimbriate surface structure which promotes attachment of the bacterium to host epithelial cells. Gonococcal pilus phase variation is characterized by a rapid on/off switch in which piliated (P+) cells throw off non-piliated (P-) variants and vice versa. Two regions of the gonococcal chromosome (pilE1 and pilE2) act as pilin expression loci, reminiscent of the MAT locus in the yeast Saccharomyces cerevisiae, while several other chromosomal regions contain silent (non-expressing) pilin sequences. Biochemical and antigenic diversity is seen in pili from a wide variety of clinical isolates. Pilins (pilus subunits) are composed of conserved N-terminal and variable C-terminal regions; the conserved region of gonococcal pilin is also found in pilins produced by widely disparate bacteria. We show here that the gonococcal pilin undergoes antigenic variation in vitro and in vivo. The protein consists of constant, semi-variable and hypervariable regions. This antigenic variation probably involves gene conversion of mini-cassettes of pilin information.     

多机泵协调排水控制问题及对策

针对实际废水排放控制系统智能程度不高和可靠性问题(在某些工况下会发生频繁切换和误切换),提出了一种基于模糊PID的协调排水控制方法,采用相位数字检测锁相技术实现机泵的50 Hz变频状态至市电工频的安全切换,给出了系统的主要硬件和软件框图.模拟实验表明,机泵切换、控制系统性能可靠,抗水位波动性强.     

基于分布结构的高效自动分拣控制系统设计

为了提高分拣系统的工作效率,设计了基于现场分布式结构和ProfibusDP总线模式的新型自动分拣控制系统。利用ProfibusDP总线将现场的所有控制可编程逻辑控器（PLC）联结成为一个整体,采用信息导轨作为控制系统向运动小车传递信息的物理载体,实现了系统所有信息的可实时传递。将运动小车进行分组,每组设立独立的控制器,提高了信息传输的速度。设计了高效的供件算法,实现了系统低速运行条件下的高速分拣作业。     

基于专家系统的异构图形数据格式转换方法研究

现在许多行业用到绘图软件,如建筑、机械、电气等,为了提高工作效率,常常需要将两种图形进行相互转换,如地理信息图与AutoCAD图形的相互转换,但是多种绘图软件之间并不为对方提供转换的接口。本文建立了一种基于专家系统的异构图形数据格式转换模型,通过图形转换综合数据库记录整个转换周期的实时状态;分析构建了图形转换知识库,保证知识库的全面性和正确性成为关键;基于正向推理策略设计图形转换推理机制。本文的创新点为将专家系统引入到异构图形转换中。解决了不同的图形数据格式之间的转换,并以油田中的双狐图形与AutoCAD图形为例,完成异构图形的转换。     

Dynamin-like protein encoded by the Drosophila shibire gene associated with vesicular traffic.

Temperature-sensitive paralysis is the most striking defect of adult Drosophila carrying the shibire mutation. This is believed to be due to a reversible block of endocytosis, which prevents membrane cycling and thus depletes synaptic vesicles. The shibire mutation also affects many tissues outside the nervous system. We have now mapped and characterized the shibire gene. A 275-kilobase yeast artificial chromosome was subcloned into cosmids, among which the gene was then located by analysing with restriction-fragment length polymorphisms. A 15-kilobase fragment of wild-type DNA rescues the mutant phenotype and the sequence of two mutant alleles show differences with wild type, demonstrating that we have isolated the shibire gene. The gene encodes a protein that is highly similar to rat dynamin, 69% of the amino-acid sequence is identical. Dynamin is a GTP-driven mechanochemical enzyme related to mammalian mx-proteins and to the yeast vps 1 gene product. Because the shibire gene product and dynamin have extensive similarity, we propose that they are cognate homologues. Dynamin causes microtubules to slide along each other in vitro and in extracts it is associated with a distinct, but so far uncharacterized, membrane fraction. In light of the shibire phenotype, we suggest that these proteins provide the motor for vesicular transport during endocytosis.     

Double-strand breaks at an initiation site for meiotic gene conversion

It has been proposed that the initiation of meiotic recombination involves either single-strand or double-strand breaks in DNA. It is difficult to distinguish between these on the basis of genetic evidence because they give rise to similar predictions. All models invoke initiation at specific sites to explain polarity, which is a gradient in gene conversion frequency from one end of a gene to the other. In the accompanying paper we describe the localization of an initiation site for gene conversion to the promoter region of the ARG4 gene of the yeast Saccharomyces cerevisiae. Here, we show that a double-strand break appears at the ARG4 recombination initiation site at the time of recombination, and that the broken DNA molecules end in long single-stranded tails.