Insertion of Robertson's Mutator in an exon affects transcript stability

Recently we reported that insertion of the 1.4 kb Mu1 element in the first intron of Adh1 affects transcription but not processing (Rowland and Strommer, Proc. Natl. Acad. Sci, USA 82:2875-2879, 1985). To determine if Mu has a similar effect at other loci, we have analyzed a mutator-induced mutant of sucrose synthetase (Sh) recovered by Don Robertson.

Using anoxic roots as a source of RNA, we have compared the Sh-RNA level in this mutant line to that of a related non-mutant line. The mutant produces less than 5% of the Sh-RNA produced by the nonmutant, and the mutant allele's gene product is approximately 1.4 kb longer than normal Sh-RNA. By Southern mapping techniques, we have localized a Mu1-like insertion within the first 40 bp of the Sh gene. Using an in vitro transcription assay, we have found transcription of the mutant Sh allele is less than transcription of the nonmutant, but this decrease could account at most for a 50% decrease in the Sh-RNA level. Therefore, in contrast to our findings with Adh1-Mu mutants, where Mul resides in an intron, insertion of Mu in an exon of Sh reduces message levels beyond the decrease associated with transcription. We conclude that this insertion of Mu within the first exon of Sh results in synthesis of an unstable transcript. Because this effect has not been seen in mutants containing Mu within introns, we hypothesize that processing of Mu-containing transcripts occurs normally but that aberrant mature transcripts are unstable.

Jeannie Rowland and Judy Strommer

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