Expression and post-transcriptional regulation of maize transposable element MuDR and its derivatives

The transposition of Mu elements underlying Mutator activity in maize requi res a transcriptionally active MuDR element. Despite variation in MuDR copy number and RNA levels in Mutator lines, transposition events are consisten tly late in plant development, and Mu excision frequencies are similar. Her e, we report previously unsuspected and ubiquitous MuDR homologs that produ ce both RNA and protein. MuDR transcript levels are proportional to MuDR co py number, and homolog transcript levels increase in active Mutator lines. A subset of homologs exhibits constitutive transcription in MuDR(-) and epi genetically silenced MuDR lines, suggesting independent transcriptional reg ulation. Surprisingly, immunodetection demonstrated nearly invariant levels of MuDR and homolog protein products in all tested Mutator and non-Mutator stocks. These results suggest a strict control over protein production, wh ich might explain the uniform excision frequency of Mu elements. Moreover, the nonfunctional proteins encoded by homologs may negatively regulate Muta tor activity and represent part of the host defense against this transposon family.