Keeping sister chromatids together: cohesins in meiosis.

Meiosis poses unique challenges to chromosome dynamics. Before entry into meiosis, each chromosome is duplicated and gives rise to two sister chromatids linked to each other by cohesion. Production of haploid gametes requires segregation of homologous chromosomes in the first meiotic division and of sister chromatids in the second. To ensure precise distribution of chromosomes to the daughter cells, sister chromatid cohesion (SCC) has to be dissolved in two steps. Maintenance and regulation of SCC is performed by the cohesin protein complex. This short review will primarily focus on the core cohesin proteins before venturing into adjacent territories with an emphasis on interacting proteins and complexes. It will also concentrate on mammalian meiosis and only occasionally discuss cohesion in other organisms.

Novel meiosis-specific isoform of mammalian SMC1.

Structural maintenance of chromosomes (SMC) proteins fulfill pivotal roles in chromosome dynamics. In yeast, the SMC1-SMC3 heterodimer is required for meiotic sister chromatid cohesion and DNA recombination. Little is known, however, about mammalian SMC proteins in meiotic cells. We have identified a novel SMC protein (SMC1beta), which-except for a unique, basic, DNA binding C-terminal motif-is highly homologous to SMC1 (which may now be called SMC1alpha) and is not present in the yeast genome. SMC1beta is specifically expressed in testes and coimmunoprecipitates with SMC3 from testis nuclear extracts, but not from a variety of somatic cells. This establishes for mammalian cells the concept of cell-type- and tissue-specific SMC protein isoforms. Analysis of testis sections and chromosome spreads of various stages of meiosis revealed localization of SMC1beta along the axial elements of synaptonemal complexes in prophase I. Most SMC1beta dissociates from the chromosome arms in late-pachytene-diplotene cells. However, SMC1beta, but not SMC1alpha, remains chromatin associated at the centromeres up to metaphase II. Thus, SMC1beta and not SMC1alpha is likely involved in maintaining cohesion between sister centromeres until anaphase II.

Mitogen-activated protein kinase phosphatase is required for genotoxic stress relief in Arabidopsis.

Genotoxic stress activates complex cellular responses allowing for the repair of DNA damage and proper cell recovery. Although plants are exposed constantly to increasing solar UV irradiation, the signaling cascades activated by genotoxic environments are largely unknown. We have identified an Arabidopsis mutant (mkp1) hypersensitive to genotoxic stress treatments (UV-C and methyl methanesulphonate) due to disruption of a gene that encodes an Arabidopsis homolog of mitogen-activated protein kinase phosphatase (AtMKP1). Growth of the mkp1 mutant under standard conditions is indistinguishable from wild type, indicating a stress-specific function of AtMKP1. MAP kinase phosphatases (MKPs), the potent inactivators of MAP kinases, are considered important regulators of MAP kinase signaling. Although biochemical data from mammalian cell cultures suggests an involvement of MKPs in cellular stress responses, there is no in vivo genetic support for this view in any multicellular organism. The genetic and biochemical data presented here imply a central role for a MAP kinase cascade in genotoxic stress signaling in plants and indicate AtMKP1 to be a crucial regulator of the MAP kinase activity in vivo, determining the outcome of the cellular reaction and the level of genotoxic resistance.

An SMC-like protein is required for efficient homologous recombination in Arabidopsis.

In plants, the observed low frequency of gene targeting and intrachromosomal recombination contrasts markedly with the efficient extrachromosomal recombination of DNA. Thus, chromatin accessibility can have a major influence on the recombination frequency of chromosomal DNA in vivo. An Arabidopsis mutant hypersensitive to a range of DNA-damaging treatments (UV-C, X-rays, methyl methanesulfonate and mitomycin C) is also defective in somatic intrachromosomal homologous recombination. The wild-type gene encodes a protein closely related to the structural maintenance of chromosomes (SMC) family involved in structural changes in chromosomes. Although loss of SMC function is lethal in other eukaryotes, growth of the Arabidopsis mutant is normal in the absence of genotoxic treatments. This suggests a surprisingly specialized function for this protein in plants, and provides the first in vivo evidence for the involvement of an SMC protein in recombinational DNA repair. It is possible that SMC-like proteins in plants alleviate suppressive chromatin structure limiting homologous recombination in somatic cells.

Involvement of Arabidopsis thaliana ribosomal protein S27 in mRNA degradation triggered by genotoxic stress.

A recessive Arabidopsis mutant with elevated sensitivity to DNA damaging treatments was identified in one out of 800 families generated by T-DNA insertion mutagenesis. The T-DNA generated a chromosomal deletion of 1287 bp in the promoter of one of three S27 ribosomal protein genes (ARS27A) preventing its expression. Seedlings of ars27A developed normally under standard growth conditions, suggesting wild-type proficiency of translation. However, growth was strongly inhibited in media supplemented with methyl methane sulfate (MMS) at a concentration not affecting the wild type. This inhibition was accompanied by the formation of tumor-like structures instead of auxiliary roots. Wild-type seedlings treated with increasing concentrations of MMS up to a lethal dose never displayed such a trait, neither was this phenotype observed in ars27A plants in the absence of MMS or under other stress conditions. Thus, the hypersensitivity and tumorous growth are mutant-specific responses to the genotoxic MMS treatment. Another important feature of the mutant is its inability to perform rapid degradation of transcripts after UV treatment, as seen in wild-type plants. Therefore, we propose that the ARS27A protein is dispensable for protein synthesis under standard conditions but is required for the elimination of possibly damaged mRNA after UV irradiation.

5'-regulatory region of Agrobacterium tumefaciens T-DNA gene 6b directs organ-specific, wound-inducible and auxin-inducible expression in transgenic tobacco.

The regulatory activity of a 826 bp DNA fragment located upstream of the pTiBo542 TL-DNA gene 6b coding region was analyzed in transgenic tobacco, using beta-glucuronidase (gus) as a reporter gene. The region was shown to drive organ-specific, wound- and auxin-inducible expression of the reporter, the effect being dependent on the type and concentration of auxin.

[Expression of a partially modified delta-endotoxin gene from Bacillus thuringiensis var. tenebrionis in transgenic potato plants]. / Ekspressiia chastichno modifitsirovannogo gena delta-éndotoksina iz Bacillus thuringiensis var. tenebrionis v transgennykh rasteniiakh kartofelia.

A modified gene (Bt77) of delta-endotoxin from Bacillus thuringiensis var. tenebrionis was constructed and cloned into pMON505. This binary transformation vector was introduced into Agrobacterium tumefaciens strains containing different helper disarmed Ti-plasmids, LBA4404, A281, and CBE21. These Agrobacterium strains were used to transform potato stem segments (S. tuberosum, cv Desiree, Resy, Temp, Granat). Regenerants were selected on kanamycin-containing media. The presence of the Bt77 sequence in plant genomic DNA was confirmed by PCR analysis. Bt gene expression was studied in regenerated plants. Western blot analysis revealed that transgenic plants produced the Bt protein in the range of 0.005-0.02% of total protein. Total protection against insect damage of leaf tissue from these plants was observed in laboratory bioassays with of Colorado beetle larvae. Transgenic plants showed incomplete protection from CB larvae.