Site-selected insertion of the transposon Tc1 into a Caenorhabditis elegans myosin light chain gene

We used the polymerase chain reaction to detect insertions of the transposon Tc1 into mlc-2, one of two Caenorhabditis elegans regulatory myosin light chain genes. Our goals were to develop a general method to identify mutations in any sequenced gene and to establish the phenotype of mlc-2 loss-of-function mutants. The sensitivity of the polymerase chain reaction allowed us to identify nematode populations containing rare Tc1 insertions into mcl-2. mlc-2::Tc1 mutants were subsequently isolated from these populations by a sib selection procedure. We isolated three mutants with Tc1 insertions within the mlc-2 third exon and a fourth strain with Tc1 inserted in nearby noncoding DNA. To demonstrate the generality of our procedure, we isolated two additional mutants with Tc1 insertions within hlh-1, the C. elegans MyoD homolog. All of these mutants are essentially wild type when homozygous. Despite the fact that certain of these mutants have Tc1 inserted within exons of the target gene, these mutations may not be true null alleles. All three of the mlc-2 mutants contain mlc-2 mRNA in which all or part of Tc1 is spliced from the pre-mRNA, leaving small in-frame insertions or deletions in the mature message. There is a remarkable plasticity in the sites used to splice Tc1 from these mlc-2 pre-mRNAs; certain splice sites used in the mutants are very different from typical eukaryotic splice sites.