Isolation of Spm controlling elements from maize

Activation of the maize transposable element Suppressor-mutator (Spm) in tissue culture

Previous experiments have revealed that the maize transposable element Activator (Ac) may become active during tissue culture. The objective of the present study was to determine whether a second transposable element, Suppressor-mutator (Spm), could also be activated in tissue culture and detected in regenerated maize plants. Approximately 500 R1 progeny of 143 regenerated plants (derived from 49 embryo cell lines) were crossed as males onto an Spm-responsive tester stock. Spm activity was observed in two R1 progeny of a single regenerated plant. This plant had been regenerated from Type II (friable embryogenic) callus of an A188 × B73 genetic background after 8 months in culture; the absence of Spm activity in four other plants regenerated from this same callus demonstrates that Spm activity was not present before culturing. Approximately 20 Spm-homologous DNA sequences were detected in each of the inbreds used to initiate the tissue cultures; it is presumed that one of these became active to give rise to Spm activity.

Sequence comparisons of three wild-type Bronze-1 alleles from Zea mays

We have sequenced genomic clones of two wild-type Bronze-1 (Bz1) alleles, and a cDNA clone from a third wild-type Bz1 allele from maize. Two overlapping transcripts initiate at least 250 bp apart. The first AUG codon after the shorter and more abundant transcript cap site(s) begins the longest open reading frame. The transcript is preceded by a putative TATA box, but not a recognizable CAAT box. The bz1 gene contains a single intron, and exhibits a strong bias for codons with the highest G+C content. Sequence polymorphisms among the Bz1 alleles include deletions/additions, a transposable element insertion, and single base pair substitutions.

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.

RNA splicing permits expression of a maize gene with a defective Suppressor-mutator transposable element insertion in an exon

The bz-m13CS9 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-m13CS9 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-1 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.

Deletions within a defective suppressor-mutator element in maize affect the frequency and developmental timing of its excision from the bronze locus

Six independent derivatives of the bz-m13 allele, which contains a 2.2-kilobase-pair defective Suppressor-mutator (dSpm) insertion at the bronze (bz) locus, have been isolated and analyzed. The derivatives were selected for alterations in the frequency and timing of somatic reversion; such derivatives have previously been analyzed genetically and designated "changes in state" by McClintock [McClintock, B. (1955) Carnegie Inst. Washington, Yearb. 54, 245-255]. All of the derivatives analyzed in the present study revert substantially later in development than the original insertion mutation and some show a very low frequency of reversion as well. All of the derivatives contain insertions at the same site as the parent bz-m13 allele. Deletions of 400-1300 base pairs were found in the dSpm elements in four of the six derivatives; the remaining derivatives could not be distinguished structurally from the original mutant allele. The results suggest that changes in the frequency and developmental timing of excision are attributable to alterations in the dSpm element. Furthermore, these data suggest that DNA sequences near the ends of the element are important for responding to the two transacting functions supplied by the transposition-competent Suppressor-mutator (Spm) element.