The organization of Physcomitrella patens RAD51 genes is unique among eukaryotic organisms

Abstract

Genetic recombination pathways and genes are well studied, but relatively little is known in plants, especially in lower plants. To study the recombination apparatus of a lower land plant, a recombination gene well characterized particularly in yeast, mouse, and man, the RAD51 gene, was isolated from the moss Physcomitrella patens and characterized. Two highly homologous RAD51 genes were found to be present. Duplicated RAD51 genes have been found thus far exclusively in eukaryotes with duplicated genomes. Therefore the presence of two highly homologous genes suggests a recent genome duplication event in the ancestry of Physcomitrella. Comparison of the protein sequences to Rad51 proteins from other organisms showed that both RAD51 genes originated within the group of plant Rad51 proteins. However, the two proteins form a separate clade in a phylogenetic tree of plant Rad51 proteins. In contrast to RAD51 genes from other multicellular eukaryotes, the Physcomitrella genes are not interrupted by introns. Because introns are a common feature of Physcomitrella genes, the lack of introns in the RAD51 genes is unusual and may indicate the presence of an unusual recombination apparatus in this organism. The presence of duplicated intronless RAD51 genes is unique among eukaryotes. Studies of further members of this lineage are needed to determine whether this feature may be typical of lower plants. Homologous recombination is essential for chromosomal segregation and participates in DNA replication and repair. The molecular mechanisms of homologous recombination have been studied intensively, mostly in bacteria and yeast. Homology recognition and exchange of homologous DNA strands have a central role in homologous recombination (reviewed in ref. 1). The paradigm of strand exchange proteins, the Escherichia coli RecA protein, is well conserved from bacteria to man, and several structural and functional homologues have been found in eukaryotes (reviewed in ref.2). The best characterized homologues are the Rad51 and Dmc1 proteins. Although Rad51 seems to be involved in mitotic as well as meiotic recombination, Dmc1 is expressed exclusively in meiosis. The presence of multiple functionally distinct proteins in eukaryotes is likely because of an ancient gene duplication followed by functional divergence early in eukaryotic species evolution (3). In addition to the original RAD51 and DMC1 genes, a series of other genes encoding proteins with low homology to Rad51 have been detected in yeast and other eukaryotes including man and other mammals (reviewed in refs. 2 and 4). The proteins of this group show similarities with Rad51, the most striking of which is that all of them contain Rad51͞RecA signatures; however, the low degree of homology [20-30% similarity (4)] to Rad51 distinguishes them from the original Rad51 protein. The group of corresponding genes contains genes that were annotated misleadingly as RAD51, e.g., RAD51B, RAD51C, and RAD51D and genes such as RAD55, RAD57, XRCC2, and XRCC3. These genes do not seem to be func...

The Chromatin Assembly Factor Subunit FASCIATA1 Is Involved in Homologous Recombination in Plants

“…For DNA gel blot analysis, nuclei prepared from Col-0, fas1-43N1IC4 651, and N1IC4 651 plants were mixed before DNase I digestion as indicated and digested as described above. After extraction, the DNA was digested with EcoRI, and fragments were separated by agarose gel electrophoresis, blotted onto a nylon membrane, and hybridized with radioactively labeled probes as described (Markmann-Mulisch et al, 2002).…”

Differential Requirements for RAD51 inPhyscomitrella patensandArabidopsis thalianaDevelopment and DNA Damage Repair

“…The procedures for preparation of P. patens genomic DNA and poly(A) þ RNA, preparation of radioactively labeled probes, DNA gel blotting, and RNA gel blotting were as described by Markmann-Mulisch et al (2002).…”

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Adaptive basis of codon usage in the haploid moss Physcomitrella patens

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