Genetic Recombination in Micromonospora

Biochemical mutants were obtained from Micromonospora chalcea, M. purpurea, and M. echinospora by using ultraviolet radiation or nitrosoguanidine. Crosses carried out between complementary nutritional mutants of the same species showed positive genetic interaction. Data are reported which indicate that the interaction between the crossed strains is due to genetic recombination. No evidence for interspecific genetic recombination was found.     

Modeling Interference in Genetic Recombination

In analyzing genetic linkage data it is common to assume that the locations of crossovers along a chromosome follow a Poisson process, whereas it has long been known that this assumption does not fit the data. In many organisms it appears that the presence of a crossover inhibits the formation of another nearby, a phenomenon known as ``interference.' We discuss several point process models for recombination that incorporate position interference but assume no chromatid interference. Using stochastic simulation, we are able to fit the models to a multilocus Drosophila dataset by the method of maximum likelihood. We find that some biologically inspired point process models incorporating one or two additional parameters provide a dramatically better fit to the data than the usual ``no-interference' Poisson model.     

Genetic Recombination in Colletotrichum sublineolum

Genetic recombination without typical parasexuality (parameiosis) was shown in the fungus Colletotrichum sublineolum, causal agent of anthracnose on sorghum. The cross among auxotrophic mutants and mutants resistant to benomyl and cycloheximide confirmed the occurrence of heterokaryosis in this species. Aneuploid and haploid recombinants were obtained from heterokaryons. Some segregants released sectors continually after several replication cycles, and were considered aneuploids in the process of haploidization. Heterokaryons derived from crosses among mutants that presented low pathogenicity produced more virulent recombinants. Parasexuality with parameiosis may represent a natural mechanism for genetic variability amplification explaining the rapid appearance of new C. sublineolum physiological races.     

Genetic Analyses of Meiotic Recombination in Arabidopsis

Meiosis is essential for sexual reproduction and recombination is a critical step required for normal meiosis. Understanding the underlying molecular mechanisms that regulate recombination is important for medical, agricultural and ecological reasons. Readily available molecular and cytological tools make Arabidopsis an excellent system to study meiosis. Here we review recent developments in molecular genetic analyses on meiotic recombination. These include studies on plant homologs of yeast and animal genes, as well as novel genes that were first identified in plants. The characterizations of these genes have demonstrated essential functions from the initiation of recombination by double-strand breaks to repair of such breaks, from the formation of doubie-HoUiday junctions to possible resolution of these junctions, both of which are critical for crossover formation. The recent advances have ushered a new era in plant meiosis, in which the combination of genetics, genomics, and molecular cytology can uncover important gene functions.     

Complementation and Genetic Recombination in Candida lipolytica

Nutritional requirements were introduced into wild-type, heterothallic strains of Candida lipolytica by exposing the cells to X rays. Complementing hybrids were recovered from mixtures of the auxotrophic strains, and genetic recombination was observed in individually isolated ascospores from the hybrid strains.     

Xer Site-Specific Recombination,an Efficient Tool To Introduce Unmarked Deletions into Mycobacteria

Genetic manipulation of mycobacteria still represents a serious challenge due to the lack of tools and selection markers. In this report, we describe the development of an intrinsically unstable excisable cassette for introduction of unmarked mutations in both Mycobacterium smegmatis and Mycobacterium tuberculosis.Mycobacterium tuberculosis causes about 2 million deaths worldwide every year (15). Over the last few years, M. tuberculosis pathogenesis characterization at a molecular level required the development of efficient genetic tools for recombination and mutagenesis. The employment of replicating temperature-sensitive and suicide plasmids (14), specialized transducing mycobacteriophages (1, 9), and a recombineering system based on two exogenous recombinases (24) improved the ability to obtain mycobacterial mutants by homologous recombination. However, the availability of only few selection markers represents a real problem when multiple knockouts are required for the study of redundant gene families in mycobacteria. A way to circumvent this problem is by the production of unmarked mutations, which can be obtained by homologous recombination and selection for sequential crossing-over events using both positive and negative markers for counterselection of the different allelic exchange events (13), or by a different approach relying on sequence-specific recombination systems allowing the excision of the positive selection marker after it has been used to select for the recombination event (19).Three different sequence-specific recombinase systems have been successfully used with mycobacteria: the TnpR/res system of the γδ transposon (1), the Flp/FRT system of Saccharomyces cerevisiae (18, 20), and the LoxP/cre systems from bacteriophage P1 (11, 19). While the Flp/FRT and the Lox/cre systems were shown to work efficiently in both slow- and fast-growing mycobacteria, the Tnp/res system was proved to be efficient only in fast-growing species. All of these systems require a first step during which the expression of an exogenous resolvase or recombinase from a replicative plasmid allows the excision of the resistant marker and a second step to eliminate the replicative plasmid, making the procedure very time-consuming, particularly when working with slow-growing mycobacteria.Recently, a new sequence-specific recombinase system based on the endogenous Xer recombinases (Xer-cise) was shown to be amenable for genetic manipulation and construction of unmarked deletion mutants in Escherichia coli and Bacillus subtilis (4). In this system, the antibiotic resistance cassette, flanked by dif sites, is intrinsically unstable since the endogenous recombinases XerC and XerD recognize and resolve the dif sites that border the cassette. This method, relying on endogenous recombinases, does not require the introduction and the subsequent removal of replicating plasmids carrying exogenous genes, making it extremely simple and practical.E. coli XerC and XerD recombinases are essential for chromosome segregation during cell division, as their role is to resolve chromosome dimers to monomers recognizing the 28-bp dif sequence present at the replication terminus region (8). The Xer site-specific recombination system is very well conserved in prokaryotes with circular chromosomes, and homologues of XerC and XerD have been identified among Gram-negative and Gram-positive bacteria (16).In this report, we adapted the Xer-cise technique to mycobacteria, demonstrating that it can be employed as a practical and efficient genetic tool for manipulating both M. tuberculosis and Mycobacterium smegmatis.     

转导引起基因重组的教学分析

转导引起基因重组的教学分析张霞朱开平（新疆石河子大学师范学院生化系,石河子８３２００３）ＡｎａｌｙｓｉｓｏｆＧｅｎｅｔｉｃＲｅｃｏｍｂｉｎａｔｉｏｎＴｈｒｏｕｇｈＴｒａｎｓｄｕｃｔｉｏｎＺｈａｎｇＸｉａＺｈｕＫａｉｐｉｎｇ（ＤｅｐａｒｔｍｅｎｔｏｆＢ...     

The Genetic Requirements for Fast and Slow Growth in Mycobacteria

Mycobacterium tuberculosis infects a third of the world''s population. Primary tuberculosis involving active fast bacterial replication is often followed by asymptomatic latent tuberculosis, which is characterised by slow or non-replicating bacteria. Reactivation of the latent infection involving a switch back to active bacterial replication can lead to post-primary transmissible tuberculosis. Mycobacterial mechanisms involved in slow growth or switching growth rate provide rational targets for the development of new drugs against persistent mycobacterial infection. Using chemostat culture to control growth rate, we screened a transposon mutant library by Transposon site hybridization (TraSH) selection to define the genetic requirements for slow and fast growth of Mycobacterium bovis (BCG) and for the requirements of switching growth rate. We identified 84 genes that are exclusively required for slow growth (69 hours doubling time) and 256 genes required for switching from slow to fast growth. To validate these findings we performed experiments using individual M. tuberculosis and M. bovis BCG knock out mutants. We have demonstrated that growth rate control is a carefully orchestrated process which requires a distinct set of genes encoding several virulence determinants, gene regulators, and metabolic enzymes. The mce1 locus appears to be a component of the switch to slow growth rate, which is consistent with the proposed role in virulence of M. tuberculosis. These results suggest novel perspectives for unravelling the mechanisms involved in the switch between acute and persistent TB infections and provide a means to study aspects of this important phenomenon in vitro.     

Genetic Recombination in Klebsiella pneumoniae An Approach to Genetic Linkage Mapping

Genetic recombination was observed between two different strains of Klebsiella pneumoniae, which is a non-motile and encapsulated bacterium belonging to the family Enterobacteriaceae and has about 55% of its DNA content as GC. The mode of recombination seemed to be similar to that of the F-factor mediated conjugation in Escherichia coli. One strain acted as the donor and the other as the recipient, and a relatively large fragment of the donor's chromosome was transferred unilaterally and unidirectionally by cell to cell contact. No genetic factor which is associated with the recombination has been identified. The genetic linkage map of K. pneumoniae was analyzed various mutants derived from the two strains. It was found that the 28 markers so far investigated were arranged linearly in a single linkage group, and that the genetic linkage map of K. pneumoniae, like that of E. coli, could be considered circular. The proposed genetic linkage map of K. pneumoniae was quite similar to that of E. coli or Salmonella typhimurium. The close similarities in this map among the three species suggest a possibility that K. pneumoniae may have differentiated from an ancestor common all three species.     

Genetic Recombination and Clonal Selection in DROSOPHILA MERCATORUM

Discrete and continuous generation unisexual populations were established from parthenogenetic D. mercatorum females heterozygous for five visible loci and one electrophoretic locus, which marked all five major chromosome arms. In the first impaternate generation of the discrete generation population, all thirty-two possible recombinant genotypes for the five visible markers were displayed. However, 99% of these individuals were homozygous for all gene markers due to the predominant mode of parthenogenesis called nuclear duplication. Many of these homozygous individuals gave rise to isogenic clones that were compelled to compete with each other in subsequent generations. A detailed analysis of the genetic response of this clonal population showed strong evidence for selection involving epistatic interactions between linked and unlinked loci throughout the genome. The unit of selection is described as being determined as early as the zygotic stage of development.     

Genetic Recombination in Streptomyces bikiniensis var. zorbonensis

A genetic recombination system in Streptomyces bikiniensis var. zorbonensis is described. This strain produces a mixture of antibiotics including zorbamycin and zorbonomycin B and C. A genetic map has been constructed from data obtained from an analysis of haploid recombinants which shows linkage relationships of 17 marker loci. Determination of map location has been made for three different loci affecting antibiotic biosynthesis in this strain.     

Genetic Analysis of Variation in Human Meiotic Recombination

The number of recombination events per meiosis varies extensively among individuals. This recombination phenotype differs between female and male, and also among individuals of each gender. In this study, we used high-density SNP genotypes of over 2,300 individuals and their offspring in two datasets to characterize recombination landscape and to map the genetic variants that contribute to variation in recombination phenotypes. We found six genetic loci that are associated with recombination phenotypes. Two of these (RNF212 and an inversion on chromosome 17q21.31) were previously reported in the Icelandic population, and this is the first replication in any other population. Of the four newly identified loci (KIAA1462, PDZK1, UGCG, NUB1), results from expression studies provide support for their roles in meiosis. Each of the variants that we identified explains only a small fraction of the individual variation in recombination. Notably, we found different sequence variants associated with female and male recombination phenotypes, suggesting that they are regulated by different genes. Characterization of genetic variants that influence natural variation in meiotic recombination will lead to a better understanding of normal meiotic events as well as of non-disjunction, the primary cause of pregnancy loss.     

Genetic Recombination in the Lignin-Degrading Basidiomycete Phanerochaete chrysosporium

Heterokaryons made from auxotrophic strains of the lignin-degrading basidiomycete Phanerochaete chrysosporium were induced to fruit. The isolation of wild-type and double-mutant phenotypes from these crosses indicated that genetic recombination had occurred. Cytological studies demonstrated that more than 90% of the basidiospores from the wild-type and auxotrophic strains and from forced heterokaryons were binucleate. Colonies of the wild-type strain of P. chrysosporium arising from single, predominantly uninucleate conidia were all capable of producing fruit bodies and basidiospores.     

Genetic Modification of Recombination Rate in TRIBOLIUM CASTANEUM

Asymmetrical responses were obtained in a replicated study of 15 generations of two-way selection for recombination rate between the ruby (rb) and jet (j) loci in Tribolium castaneum. Recombination rates in the two replicate high lines increased from an average of 0.22 in the base populations to an average of 0.42 at generation 15. Recombination rate pooled over the 15 generations of selection in each low line was significantly less than the control but there was no clear downward trend in response to selection for decreased recombination rate. The realized heritabilities were 0.16 +/- 0.03 and 0.17 +/- 0.02 in the two high lines, and were not significantly different from zero in the two low lines. Selection was based on crossing over in cis females only; however, rates measured in cis males after 12 generations showed the same response patterns as female rates. Similar response patterns were also determined for recombination measured in trans males and females at generation 18 following three generations of relaxed selection. The distribution of recombination rates measured in backcross beetles [(H X L) X H and (H X L) X L] at generation 12 indicated polygenic control with those genes decreasing recombination rate being dominant. Detailed analysis of recombination rates in F1's produced by interline crosses at generation 15 confirmed the directional dominance findings. Under a polygenic model of recombination modifiers in which low recombination is dominant to high, average recombination rates will increase as inbreeding progresses, thus providing a mechanism for the production of new gene combinations in small populations.