Worm Breeder's Gazette 12(2): 59 (January 1, 1992)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

Unexpected splicing of mlc-2 ::T c1 mRNAs

Alice Rushforth, Phil Anderson

Laboratory of Genetics, University of Wisconsin, Madison, WI.

As previously described (WBG 11(5):65) we have used the polymerase chain reaction (PCR) in combination with a sib-selection procedure to isolate Tc1 transposon insertions into targeted genes. Using this procedure we have isolated three independent transposon-induced mutations in the myosin regulatory light chain gene mlc-2 .We sequenced the DNA across the insertional junctions in each mutant. Two insertions, mlc-2 ( r926 ::T c1 )and mlc-2 ( r927 ::T c1 ),are located at an identical position (but in opposite orientations) 6 bp upstream of the 5' splice donor within the mlc-2 third exon. The third insertion, mlc-2 ( r948 :T c1 )is located at position 2051 within the third exon of mlc-2 .All three mlc-2 ::T c1 mutants are wild-type as homozygotes.

By Northern analysis, all these mutants accumulate somewhat reduced amounts of an approximately wild-type sized mlc-2 mRNA. We sequenced the mutant and wild-type cDNAs in the region of the Tc1 insertions. We previously reported (Rushforth et al., 1990 C. elegans Meetings Abstracts) that for mlc-2 ::T c1 mutants r926 and r927 a cryptic 5' splice donor (18 nt upstream of the wild-type 5' splice donor) is activated. Utilization of this upstream 5' splice donor removes all of Tc1 plus 18 bp of mlc-2 from the mature mRNA. The resulting mRNA can be translated in-frame to yield a mlc-2 protein deleted for six amino acids. We have subsequently determined that for mutant mlc-2 ( r948 ::T c1 )a 5' splice donor within the left inverted repeat of Tc1 (at position 38) is spliced to a 3' splice acceptor within the right inverted repeat of Tc1 (at position 1601). Utilization of these splice sites removes most of Tc1 ,leaving 46 nt of the transposon plus the TA target site duplication (for a total of 48 nt), from the mature mlc-2 mRNA. This 48 nt insertion contains no stop codons. Therefore, the resulting mRNA can be translated in-frame to yield a mlc-2 protein with an insertion of 16 amino acids. Because these mlc-2 ::T c1 mutants are wild-type in phenotype we do not know whether the mutant mlc-2 proteins are functional or non-functional.

Thus, all three of our mlc-2 ::T c1 insertions generate translatable mRNAs using a variety of novel splice sites. We do not know if splicing to remove Tc1 from mRNAs is unique to mlc-2 (or some subset of messages) or if it is a general property of Tc1 -inducedalleles. To address this question we plan to examine the structures of mRNAs from Tc1 -inducedalleles of other genes. Most previously analyzed Tc1 -inducedmutations were identified because they caused functional defects. It is possible that many messages containing Tc1 are spliced to yield functional protein products. If this is true it may be difficult to isolate loss-of-function mutants using this PCR/sib-selection procedure. However, mutants could subsequently be isolated by site directed mutagenesis, as proposed by Plasterk et al. (WBG 11(5):44). An altered transgene could be reintroduced at the site of insertion via DNA gap repair following Tc1 excision.