Alternative splicing in the Caenorhabditis elegans DNA topoisomerase I gene

5'-end cDNA fragments of the Caenorhabditis elegans DNA topoisomerase I gene were obtained by rapid amplification of the cDNA ends from C. elegans mRNAs. The presence of a SL1 sequence at the 5'-terminus of the cDNA sequence suggested trans-splicing of the pre-mRNA. By comparing the complete cDNA sequence with the genomic lambda DNA clones, the gene structure composed of five exons was established. Alternative splicing deleting the second exon was observed in the cDNA fragments obtained by a gene-specific reverse transcription followed by polymerase chain reactions. The shorter mRNA missing the second exon was expressed at all the developmental stages, while the full-length mRNA was present only in embryos.