The maize Adh1-S low expression mutant S3034 contains a 1.4 kb insertion, designated Mul, in the first intron. The reduction in ADH1 enzymatic activity has been correlated with reduced mRNA levels (Strommer et al, Nature 300:542-544, 1983); and the decreased mRNA level has been shown to be due to decreased levels of transcription (Rowland and Strommer, Proc. Natl. Acad. Sci. USA 82:2875-2879, 1985).
To determine whether the decrease in transcriptional activity is correlated with altered packaging of the DNA, we have been analyzing Adh1 chromatin structure by means of DNAse I digestion of nuclei extracted from seedling roots. To control for transacting variability, Adh1-S/S3034 heterozygotes have been used as a source of nuclei. The restriction fragment polymorphism conveyed by the Mu1 insert provides us with a direct quantitative comparison of the general DNAse I sensitivity of the two alleles.
Our results indicate that there is a three- to five-fold reduction in the DNAse I sensitivity of the S3034 allele compared to the Adh1-S allele. It appears, therefore, that insertion of Mu1 into the first intron has changed the chromatin structure of the region flanking it, rendering the DNA less accessible to degradative enzymes. The alteration inhibiting DNAse I susceptibility may lead to impaired binding or transcription by the RNA polymerase complex and thereby account at least in part for decreased transcription of the mutant allele.
Daniel Ortiz and Judy Strommer
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