Characterization of the major transcripts encoded by the regulatory MuDR transposable element of maize

The MuDR element controls the transposition of the Mutator transposable element family in maize. Previous studies reported the presence of two major MuDR-homologous transcripts that correlate with Mutator activity. In this study, we describe the structure and processing of these two major transcripts. The transcripts are convergent, initiating from opposite ends of the element within the 220-bp terminal inverted repeats. The convergent transcripts do not overlap, and only 200 bp of internal MuDR sequences are not transcribed. Cloning and sequencing of multiple MuDR cDNAs revealed unusual intron/exon junctions, differential splicing, and multiple polyadenylation sites. RNase protection experiments indicated that some splicing failure occurs in young seedlings, and that a low level of antisense RNA exists for both transcripts. On a whole plant level, the presence of the major MuDR transcripts strictly correlates with Mutator activity in that no MuDR transcripts are observed in non-Mutator or inactive Mutator stocks. Examination of various tissues from active Mutator stocks indicates that the two transcripts are present in all organs and tissues tested, including those with no apparent transposition activity. This suggests that Mutator activity is not simply controlled by the level of the major MuDR transcripts.