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 λ 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.