Isolation of human and mouse genes based on homology to REC2, a recombinational repair gene from the fungus  Ustilago maydis

Abstract

A human and a mouse gene have been isolated based on homology to a recombinational repair gene from the corn smut Ustilago maydis. The new human (h) gene, termed hREC2, bears striking resemblance to several others, including hRAD51 and hLIM15. hREC2 is located on human chromosome 14 at q23-24. The overall amino acid sequence reveals characteristic elements of a RECA-like gene yet harbors an src-like phosphorylation site curiously absent from hRAD51 and hLIM15. Unlike these two relatives, hREC2 is expressed in a wide range of tissues including lung, liver, placenta, pancreas, leukocytes, colon, small intestine, brain, and heart, as well as thymus, prostate, spleen, and uterus. Of greatest interest is that hREC2 is undetectable by reverse transcription-coupled PCR in tissue culture unless the cells are treated by ionizing radiation.The RecA protein of Escherichia coli provides the enzymatic function for recognition of DNA sequence homology and DNA strand exchange. These are essential steps in the molecular systems necessary for genetic recombination and recombinational repair. recA-like genes are ubiquitous in prokaryotes, and the pairing mechanism derived from studies on the E. coli RecA protein is the prototype for this class of enzymes. The discovery that the Rad51 protein of Saccharomyces cerevisiae is a RecA structural homologue and is required for recombination and repair proficiency has provided strong evidence in favor of the universality of the RecA paradigm (1-3). This view has been strengthened by the discovery of Rad51 homologues in organisms representing numerous groups of eukaryotes including human.One puzzling aspect of the genetic control of recombination and repair in eukaryotes is the multiplicity of genes encoding RecA-like proteins. In S. cerevisiae a total of four genes encoding proteins with structural homology to RecA have been discovered. In addition to RAD51 these include RAD55 (4), RAD57 (5), and DMC1 (6), the latter of which is expressed only during meiosis. RAD51, RAD55, and RAD57 are expressed mitotically, as well as meiotically, but are not redundant in function. Inactivation of any one of these genes leads to loss of proficiency in DNA repair and to defective meiosis.Two mitotically expressed genes encoding proteins with RecA homology have been found in the fungus Ustilago maydis. One (UmRAD51) encodes a protein very close in structure to the human Rad51 protein (7), and the other (REC2) encodes a protein more than twice as large as HsRad51 but nonhomologous over most of the 200-amino acid region corresponding to the ''homologous core'' residues of RecA protein, except for a 50-amino acid stretch of conserved residues that is essential for activity in recombination and repair (8). Nonetheless, biochemical studies have shown that Rec2 protein, the product of this structurally divergent gene, is active in promoting RecA-like DNA pairing reactions in vitro (9). The relationship between the U. maydis REC2 and RAD51 genes is by no means clear, but genetic analysis has indicated so...

Gene targeting is enhanced in human cells overexpressing hRAD51

“…In this respect, it is interesting to note that the mammalian RAD51 gene family contains at least seven different genes whose specific functions are under investigation. 20,[42][43][44][45][46] It is possible that different homologous recombination processes make use of variable sets of gene products, or that accessory proteins are present at variable levels. hRAD51 overexpression might only enhance gene targeting threefold before another component of the recombination machinery becomes limiting, and this could be different for intrachromosomal recombination.…”

Complex Formation by the Human Rad51B and Rad51C DNA Repair Proteins and Their Activities in Vitro

“…The Rad55⅐Rad57 complex acts as a cofactor for the assembly of Rad51 onto ssDNA and functions to promote the DNA strand transfer activity of Rad51 (12). In human cells, in addition to meiotic Dmc1 protein, five Rad51 paralogs have been identified: Rad51B/Rad51L1 2 (13)(14)(15), Rad51C/Rad51L2 (16), Rad51D/Rad51L3 (15,17,18), XRCC2 (19 -21), and XRCC3 (19,22,23). These Rad51 paralogs share 20 -30% sequence identity with Rad51 and with each other.…”

Efficient rejoining of radiation-induced DNA double-strand breaks in vertebrate cells deficient in genes of the RAD52 epistasis group

“…(d) Induction and rejoining of DNA DSBs in irradiated RAD52 7/7 cells. (e) Induction and rejoining of DNA DSBs in irradiated RAD54 7/7 cells tumorigenesis (Schoenmakers et al, 1999), and is induced by ultraviolet light and IR suggesting an involvement in DNA damage response (Peng et al, 1998;Rice et al, 1997). Analysis of RAD51B 7/7 DT40 cells indicates mild radiosensitivity to IR and suggests distinct roles from the RAD51 gene and other genes of the RAD52 epistasis group of genes (Takata et al, 2000).…”