Cloning and gene map assignment of the Xiphophorus DNA ligase 1 gene

Fishes represent the stem vertebrate condition and have maintained severalgene arrangements common to mammalian genomes throughout the 450 Myr ofdivergence from a common ancestor. One such syntenic arrangement includesthe GPI-PEPD enzyme association on Xiphophorus linkage group IV and humanchromosome 19. Previously we assigned the Xiphophorus homologue of thehuman ERCC2 gene to linkage group U5 in tight association with the CKMlocus. CKM is also tightly linked to the ERCC2 locus on human chromosome19, leading to speculation that human chromosome 19 may have arisen byfusion of two ancestral linkage groups which have been maintained infishes. To investigate this hypothesis further, we isolated and sequencedXiphophorus fish genomic regions exhibiting considerable sequencesimilarity to the human DNA ligase 1 amino acid sequence. Comparison of thefish DNA ligase sequence with those of other species suggests several modesof amino acid conservation in this gene. A 2.2-kb restriction fragmentcontaining part of an X. maculatus DNA ligase 1 exon was used in backcrosshybrid mapping with 12 enzyme or RFLP loci. Significant linkage wasobserved between the nucleoside phosphorylase (NP2) and the DNA ligase(LIG1) loci on Xiphophorus linkage group VI. This assignment suggests thatthe association of four DNA repair-related genes on human chromosome 19 maybe the result of chance chromosomal rearrangements.