Worm Breeder's Gazette 15(5): 38 (February 1, 1999)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

Characterization of Ce-rdh-1 (RAD51, DMC1/LIM15 homolog 1) involved in meiotic recombination

Takako Takanami1, Shusei Sato2, Takeshi Ishihara3, Isao Katsura3, Hideyuki Takahashi1, Atsushi Higashitani1,4

1 Institute of Genetic Ecology, Tohoku University, 2-1-1 Katahira, Sendai, 980-8577, Japan
2 Kazusa DNA Research Institute, 1532-3 Yana, Kisarazu, 292-0812, Japan
3 National Institute of Genetics, 1-111 Yata, Mishima, 411-8540, Japan
4 ahigashi@ige.tohoku.ac.jp

Genetic recombination is an essential process for both recombinational repair of damaged DNA and variability of the genome in sexual reproduction. In Escherichia coli, the RecA protein plays key roles in the genetic recombination by finding homologous stretches of nucleotide sequences between two DNA molecules and promoting strand exchange. In several species of eukaryote, two types of RecA-like proteins, Dmc1/Lim15 and Rad51, have been identified. The DMC1/LIM15 type genes are expressed exclusively in meiotic cells, participating in the repair of double-strand breaks at hotspots of meiotic recombination. The RAD51 type genes are expressed in both mitotic and meiotic cells, participating in recombinational repair of double-strand breaks.

Blast search analysis of the C. elegans genome project database revealed only one homolog of the recA-like gene. It was located in the cosmid clone H36F17, corresponding to map position 4.6 on chromosome IV. An EST cDNA clone yk401c3 of this gene has already been isolated by Dr. Yuji Kohara (NIG, Japan). We determined the nucleotide sequence of the cDNA clone yk401c3 by chain-terminator sequencing. The total length of the cDNA was 1380 bp and the putative gene product consisted of 357 amino acid residues (accession # AB011382). The putative product showed a high degree of sequence similarity to both the RAD51 type and the DMC1/LIM15 type genes previously reported. The C. elegans gene corresponding to the cDNA was termed as Ce-rdh-1 (C. elegans RAD51, DMC1/LIM15 homolog 1). It is probably identical to CeRAD51 (Rinaldo & La Volpe WBG 15, (2) 39).

The results of RNAi indicated that repression of Ce-rdh-1 blocked chromosome condensation of six bivalents and produced abnormal chromosomes in oocytes of F1 progeny. Oogenesis did not proceed to the diakinesis stage. Accordingly, all the eggs produced (F2) died in early stages (Takanami et al . DNA Res. 5, in press). By using RT-PCR, the following results were obtained: 1)the level of mRNA of the Ce-rdh-1 was increased in young-adult hermaphrodites; 2)the expression pattern was similar to that of a female germ cell-specific tumor suppressor gene, gld-1; 3)the expression level was extremely lower in adult males than that in hermaphrodites; 4)the expression was not induced 90 min after treatment with MMS (methyl methanesulfonate). It has been shown that frequency of meiotic recombination in male decreases by approximately two-third compared with that in hermaphrodite (Zetka & Rose Genetics 126, 355-363, 1990). From all together, the Ce-rdh-1 is concluded as a functional homolog of DMC1/LIM15 type gene participating in the meiotic recombination.

In the current C. elegans database, we could not find any additional recA-like gene and any homolog of RAD52 whose product forms a stable complex with RAD51. It has been known that the gene targeting by homologous recombination is very difficult in C. elegans. This phenomenon may be a result of loss of RAD52 epistasis group during its evolution.