Deletion Derivatives of the MuDR Regulatory Transposon of Maize Encode Antisense Transcripts but Are Not Dominant-Negative Regulators of Mutator Activities

Abstract

The maize MuDR/Mu transposable elements are highly aggressive, and their activities are held in check by host developmental and epigenetic mechanisms. The Mutator regulatory element, MuDR , produces both sense and antisense transcripts. We have investigated the impact of the presence of antisense transcripts on the abundance of the corresponding sense messages and on the regulation of Mutator activities. We report that internal deletions in MuDR arise frequently in somatic tissues; preferential loss of the 3 untranslated region of mudrA and/or mudrB containing the intergenic region is correlated with chimeric sense mudrA /antisense mudrB and sense mudrB /antisense mudrA transcripts. Heritable internal deletions are extremely frequent ( Ͼ 10 Ϫ 2 per element), and the resulting defective MuDR elements also encode antisense transcripts. Expression of endogenous or additional transgene-encoded antisense transcripts neither decreases sense transcript levels nor inhibits Mutator excision activity over the three generations examined. We propose that antisense transcripts produced by MuDR deletions are not dominant-negative regulators of Mutator activities.

MuDR Transposase Increases the Frequency of Meiotic Crossovers in the Vicinity of a Mu Insertion in the Maize a1 Gene

“…In the current study, the CO/NCO ratio observed in plants that lacked MuDR tain deletions of sequences adjacent to Mu elements and internally deleted Mu elements is consistent with was 1.8. This dramatic difference in the CO/NCO ratio between the two studies cannot be attributed to differthe existence of MuDR-catalyzed DNA breaks in the vicinity of Mu elements (Levy et al 1989; Levy and sase must at least be creating a local environment that is more conducive to the formation of endogenous DNA Walbot 1991;Lisch et al 1995;Hsia and Schnable 1996;Asakura et al 2002;Kim and Walbot 2003 events, Figure 1); or DSB repair, using as template the sister chromatid. This latter process would not generate (Brown et al 1989b) and between 4.9 ϫ 10 Ϫ6 and 2.3 ϫ 10 Ϫ3 (Schnable et al 1989).…”

“…Consistent with these recombinant chromosomes and would in fact regenerate the parental a1-mum2 allele or either internal or results, we and others (Lisch et al 1995) have shown that the rate of class VI NCOs at a1-mum2 is low. It adjacent deletions of Mu1 if gap repair is interrupted (Lisch et al 1995;Hsia and Schnable 1996; Asakura is puzzling that in this heterozygote, although MuDR increases the rate of CO fourfold, the rate of class VI et al 2002;Kim and Walbot 2003).…”

Mutator and MULE Transposons

“…Presumably, these structural features inhibit the production of excessive antisense transcripts. However, despite this, antisense transcripts due to read-through transcription or from deletion derivatives of MuDR have often been detected and do not appear to have negative effects on Mutator activity (13,14,15,16). The mudrA transcript includes three introns, one of which is in the 5′ UTR (Fig.…”

“…The presence of this antisense mudrA transcript has, however, no obvious effect on activity. Similarly, there is evidence that full length MuDR elements, due to the convergent transcription of mudrA and mudrB, produce detectable amounts of antisense transcript that do not affect levels of activity, nor do constructs specifically designed to transcribe only antisense mudrA transcript (14,16). Thus, it would appear that MuDR elements are refractive to epigenetic silencing via antisense transcript.…”

“…In MuDR, it could be that a similar source of small RNAs could be produced via read-through of the two convergently transcribed genes encoded by MuDR. It should be noted, however, that even a single MuDR element produces very large amounts of transcript, and that even 35-S driven antisense mudrA transcript does not appear to trigger MuDR silencing (14,15).…”