University of Georgia

Background effects on the expression of Mutator-induced mutant alleles

--Daniel Ortiz, Robert Gregerson and Judith Strommer

The mutant allele of alcohol dehydrogenase designated Adh1-S3034 was produced by insertion of a Mu1 element into the first intervening sequence of the progenitor Adh1-S allele. It was shown to produce 30-40% normal levels of ADH1-S peptide, while a derivative, Adh1-S3034b, was reported to exhibit three-fold lower levels of ADH1-S (Freeling et al., Devel. Genet., 1982). Despite the differences in expression, we found the two alleles indistinguishable at the level of Southern mapping. A comparison at the DNA sequence level seemed likely to provide a means of learning how minor changes in intervening sequences could result in dramatically different levels of gene expression.

We therefore cloned the S3034b allele for comparison to the previously cloned and sequenced S3034. Our surprising finding was that DNA sequences of the two putative alleles are indistinguishable. Re-examination of allozyme ratios revealed that levels of ADH1 produced by both "alleles" are highly variable and also indistinguishable. We conclude that S3034 and S3034b are the same allele.

The survey of allozyme ratios did reveal a correlation between genetic background and level of Adh1-Mu expression (relative to expression of the progenitor allele). Mu disruption of the first intervening sequence of Adh1 has a much greater effect in a background contributed by a Boone County White line than in a background contributed by Funk Fast, for example. The same pattern was found to apply to an independent mutant allele, Adh1-S4477, produced by insertion of Mu1 a few hundred nucleotides downstream from the site of insertion in S3034.

The background effect is attributable to differences in levels of steady-state RNA. It is not related to differences in DNA methylation. Preliminary evidence is that few loci are responsible for the observed variation.

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