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Five members of the RecQ helicase family, RECQL, WRN, BLM, RTS and RECQL5, have been found in human and three of them (WRN, BLM and RTS) were disclosed to be the genes responsible for Werner, Bloom and Rothmund-Thomson syndromes, respectively. RECQL5 (RecQ helicase protein-like 5) was isolated as the fifth member of the family in humans through a search of homologous expressed sequence tags. The gene is expressed with at least three alternative splicing products, alpha, beta and gamma. Here, we isolated mouse RECQL5 beta and determined the DNA sequence of full-length cDNA as well as the genome organization and chromosome locus. The mouse RECQL5 beta gene consists of 2949 bp coding 982 amino acid residues. Comparison of amino acid sequence among human (Homo sapiens), mouse (Mus musculus), Drosophila melanogaster and Caenorhabditis elegans RECQL5 beta homologs revealed three portions of highly conserved regions in addition to the helicase domain. Nineteen exons are dispersed over 40 kbp in the genome and all of the acceptor and donor sites for the splicing of each exon conform to the GT/AG rule. The gene is localized to the mouse chromosome 11E2, which has a syntenic relation to human 17q25.2-q25.3 where human RECQL5 beta exists. Our genetic characterizations of the mouse RECQL5 beta gene will contribute to functional studies on the RECQL5 beta products.

Five members of the RecQ helicase family, RECQL, WRN, BLM, RECQL4 and RECQL5 have been identified in humans. WRN and BLM have been demonstrated to be the responsible genes in Werner and Bloom syndromes, respectively. RECQL4 (RecQ helicase protein–like 4) was identified as a fourth member of the human RecQ helicase family bearing the helicase domain, and it was subsequently shown to be the responsible gene in Rothmund–Thomson syndrome. Here, we isolated mouse RECQL4 and determined the DNA sequence of full-length cDNA as well as the genome organization and chromosome locus. The mouse RECQL4 consists of 3651 base pairs coding 1216 amino acid residues and shares 63.4% of identical and 85.8% of homologous amino acid sequences with human RECQL4. The RECQL4 gene was localized to mouse chromosome 15D3 distal-E1 and rat chromosome 7q34 proximal. They were mapped in the region where the conserved linkage homology has been identified between the two species. Twenty-two exons dispersed over 7 kilo base pairs and all of the acceptor and donor sites for splicing of each exon conformed to the GT/AG rule. Our observations regarding mouse RECQL4 gene will contribute to functional studies on the RECQL4 products.

We established the radiosensitive cell line SX9 from mammary carcinoma cell line FM3A. In SX9 cells a defect of DNA-dependent protein kinase (DNA-PK) activity was suggested. Additionally, a complementation test suggested that the SX9 cell line belongs to a x-ray cross-complementing group (XRCC) 7. Isolation and sequence analyses of DNA-dependent protein kinase catalytic subunit (dna-pkcs) cDNA in SX9 cells disclosed nucleotide “T” (9572) to “C” transition causing substitution of amino acid residue leucine (3191) to proline. Interestingly, the mutation occurs in one allele, and transcripts of the dna-pkcs expressed exclusively from mutated allele. V(D)J recombination assay using extrachromosomal vector revealed the defects of not only coding but also signal joint formation. The frequency of the signal joint decreased to approximately one-tenth and the fidelity drastically decreased to 12.2% as compared with the normal cell line. To confirm the responsibility of thedna-pkcs gene for abnormal V(D)J recombination in SX9, the full-length dna-pkcs gene was introduced into SX9. As a result, restoration of V(D)J recombination by wild typedna-pkcs cDNA was observed. SX9 is a noveldna-pkcs-deficient cell line.