The bz-mJ3CS9 allele of the bronze-1 gene in maize contains a 902-base-pair defective Suppressor-mutator (dSpm) transposable element in the second exon. Nevertheless, 40-50% of the enzymatic activity conditioned by a nonmutant allele at the bronze-1 locus is routinely recovered in crude extracts prepared from plants carrying bz-mJ3CS9 in the absence of an autonomous Suppressor-mutator element. Analyses of RNAs produced by such plants show that transcription proceeds through the dSpm. The dSpm sequence of the messenger RNA precursor is then removed by RNA splicing using the donor site of the single bronze-i intron and an acceptor site within the inverted terminal repeat of the dSpm. This results in a messenger RNA with the proper reading frame that could produce a functional enzyme. These data demonstrate that this dSpm insertion in an exon of a structural gene has produced a functional allele with a novel intron consisting, in part, of the dSpm. This mechanism appears to allow dSpm elements to reduce the impact of their insertions on gene expression.The maize defective Suppressor-mutator (dSpm) transposable elements are nonautonomous members of the Suppressor-mutator (Spm; also known as Enhancer-Inhibitor, En-I) family (1-3). They can transpose or excise only in the presence of an autonomous Spm element. One of the interesting features of dSpm elements, first noted by McClintock (2,4), is that association of these elements with structural genes in some cases gives rise to nonmutant phenotypes in the absence of Spmn. In order to understand the underlying mechanism of this phenomenon, we have analyzed an allele of this type at the maize bronze-1 locus.The bronze-1 locus is one of the many loci involved in the anthocyanin biosynthetic pathway of maize. The gene encodes UDP-glucose :flavonoid 03-D-glucosyltransferase (UFGTase, EC 2.4.1.91; refs. 5 and 6). Recessive alleles at the locus result in the bronze coloration of the aleurone layer and the brown coloration of many other plant parts as compared with the wild-type purple coloration. The mutant allele analyzed in the present study has a dSpm insertion in Bz, a nonmutant allele at the bronze-1 locus (7). This mutant allele has been designated bz-ml3CS9 (abbreviated as CS9). In the presence of an active Spm, expression of CS9 is suppressed in most cells except for infrequent revertant cell lineages. As a result, one observes small colored sectors on a bronze background. In the absence of an active Spmn, CS9 conditions full anthocyanin pigmentation.In this paper, we report that CS9 conditions a relatively high level of UFGTase activity in the absence of Spin despite the presence of the dSpm element in the second exon. Analyses of RNAs produced by CS9 in the absence of Spm show that the dSpm element contains an acceptor site for RNA splicing within its inverted terminal repeat. This enables RNA splicing to efficiently remove the dSpm sequence from the mRNA precursor, allowing for gene function.

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