Alternative 3' splice acceptor sites modulate enzymic activity in derivative alleles of the maize bronze1-mutable 13 allele.

Abstract

The defective Suppressor-mutator (dSpm)-induced allele bronzel-mutable 73 (bzl-ml3) and many of its derivative alleles are leaky mutants with measurable levels of flavonol O3-gIucosyltransferase activity. This activity results from splicing at acceptor site-1, one of two cryptic 3'splice sites within the dSpm insertion in bzl-ml3-In this study, splicing in bz7-11173 change-in-state (CS) alleles CS-3 and CS-64 was shown to be altered from bz7-m73; previous work found altered splicing in CS-9. CS-64 is a null allele and lacks the acceptor site-1-spliced transcript because this site is deleted. CS-3 and CS-9 had increased levels of the acceptor site-1 transcript relative to bz7-ml3 and increased enzymic activities. A deletion in CS-9 altered splicing by eliminating acceptor site-2. Both acceptor sites were intact in CS-3, but a deletion removed most of a 275-bp GC-rich sequence in dSpm. This suggests that GC-rich sequences affect splicing and is consistent with models postulating a role for AU content in the splicing of plant introns. Splicing does not necessarily occur, however, at the junction of AU-rich intron sequences and GC-rich exon sequences.

Transposons and Gene Creation

“…Mobile elements have survived, in part, because they have evolved mechanisms to minimize their negative impacts on the genomes in which they are found. An example of this strategy is the presence of splice donor signals near the ends of members of the hAT, CACTA, and Mutator superfamilies and of the Tnt1B family of retrotransposons, allowing nearly all the transposon sequence to be spliced from the transcripts of genes into which it inserts (Kim et al, 1987;Wessler et al, 1987;Menssen et al, 1990;Ortiz and Strommer, 1990;Okagaki et al, 1992;Leprince et al, 2001). This mechanism reduces the negative selective pressure on transposon insertion, allowing a higher number of insertion events to retain expression of the target gene, though of a mutant allele.…”