Molecular characterization of suppressor-mutator (Spm)-induced mutations at the bronze-1 locus in maize: the bz-m13 alleles

The bz-m13 allele of maize contains a defective Suppressor-mutator (dSpm) transposable element and gives rise to a variety of stable and unstable derivatives in the presence of an autonomous Suppressor-mutator (Spm) element. The dSpm-13 element of bz-m13 consists of 2,241 base pairs (bp) and is located within the second exon of the bronze-1 (bz) gene. A number of the stable derivatives, both functional and nonfunctional, derived from bz-m13 were characterized molecularly. Results from genomic DNA blotting experiments indicate that the dSpm-13 element had excised from the locus in each stable derivative analyzed. The unstable derivatives bz-m13CS9 and bz-m13CS6 contain dSpm elements in the same position and orientation as the dSpm-13 element, but they differ in the length of the element. The dSpm-13CS9 element is 902 bp and arose via a deletion between two 5-bp direct repeats within the dSpm-13 element. The dSpm-13CS6 element is 2,239 bp and only differs from dSpm-13 by a 2-bp deletion at the end of one of the 13-bp terminal inverted repeats. The effect of these deletions on the frequency and timing of Spm-induced excision is discussed herein. In the absence of Spm, each of the bz-m13 alleles conditions a nonmutant phenotype despite the presence of the insertions in the second exon. The role of RNA splicing in this phenomenon and the recent finding of an acceptor splice site within the terminal inverted repeat are also discussed.