Plant RecQ-Helicase Genes, their Conservation and Function in Homologous Recombination

RecQ-helicases are present in all organisms  investigated so far.The number of genes varies from one ancestral RecQ gene in eubacteria  to seven genes in eukaryotes, which are quite different from an evolutionary and functional point of view. For several RecQ helicases a prominent role in homologous recombination (HR) has been  shown previously. The most extensively  investigated proteins in this respect are SGS1 from yeast and its functional homologues in humans and plants (BLM and RECQ4A). In the animal kingdom five different RecQ-helicases exist, and for each of them a homologous gene in land plants can be identified. In the whole plant kingdom four (in green algae) to seven (in most land plants) different genes exist, all of them showing a clear phylogenetic relationship in plants. One of the earliest land plants, the moss Physcomitrella patens (Ppa), has lost two of these seven genes (RECQ1 and 3) but possesses a duplicated RECQsim gene. The PpaRECQ4 gene, when transformed and expressed in Arabidopsis thaliana (Ath), shows full phenotypic complementation of AthRECQ4A mutants. Athrecq4A shows enhanced replication coupled HR and increased sensitivity towards intercalating genotoxic agents like cisplatin or MMC. Both phenotypes are fully reverted by expression of the moss protein RECQ4 in Arabidopsis. Therefore, the function of plant RECQ4 in suppression of HR has been conserved for at least 450 million years in plants and, beyond this, a HR suppressing function was shown earlier in animals and fungi. Summarizing all these data, the last common ancestor of eukaryotes must have possessed several different RecQ genes.