THE GENE STRUCTURE of CAENORIIABDITIS ELEGANS DNA TOPOISOMERASE I

The c-DNA sequence of DNA topoisomerase I was determined using cDNA clones isolated from C. elegant cDNA libraries and also cDNA fragments obtained by RT-PCR of C elegans mRNA. The cDNA sequence is 3.2 kb long and has an open reading frame of 806 amino acids. The open reading frame has 56% identity to the amino acid sequence of human DNA topoisomerase f. The 72 kDa (> terminal polypeptide expressed in Excherichia coli cells showed caUilytic DNA topoisomerase I activity. A developmental northern analysis showed that rnRNA level of the DNA topoisomerase I was at maximum in embryos. Several genomic DNA clones of the DNA toix>isomerase I were obtained by screening a C. elegans \genomic DNA library. By analyzing the genomic DNA clones with respect to the cDNA sequence, the gene structure containing 5 exons was delineated. The pre-rnRNA of 5 exons is alternatively spliced to produce another mRNA missing the second exon. The mRNA missing the second exon was expressed at the level twice as high as the full-length mRNA in C. elegans, and relative levels of the two mRNAs did not change with development. The pre-mRNA is trans-spliced to SL1 RNA at the 5'-end as is the case with the majority of C. elegans mRNAs. The DNA topoisomerase 1 gene was mapped to position 18 of chromosome I by screening polytenc YAC plasmid DNAs.