The organization of Physcomitrella patensRAD51 genes is unique among eukaryotic organisms.

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.