The effect of insertion of the maize transposable element mutator is dependent on genetic background

A secondary mutant, derived from an allele of maize alcohol dehydrogenase 1 (Adh1) carrying a Mutator transposable element (Mu1) in its first intron, was reported to exhibit a threefold decrease in ADH enzymatic activity and steady-state RNA levels compared to the original mutant. The original mutant, Adh1-S3034 (abbreviated S3034), was previously characterized at the molecular level. The derivative, abbreviated S3034b, has now been cloned; at the DNA sequence level the insertion and surrounding Adh1 sequences are indistinguishable from S3034. Furthermore, in our lines there is no difference in relative ADH activities between products of the two putative alleles. A comparison of gene expression in heterozygotes obtained by crossing to different tester lines reveals a correlation between the measured decrease in levels of ADH polypeptide produced by the mutant allele and the background in which it is measured; this effect is distinct from any background-related variation in the expression of the progenitor allele. It does not appear to be attributable to alternative patterns of DNA modification. It appears to reflect a background-associated difference in the level of normal Adh1-RNA produced. Thus the previously reported distinction between S3034 and S3034b may be due to differences in the extent to which the mutant allele and a given genetic background interact to produce functional Adh1-RNA.