Author(s): Thomas P. Howard 1, Andrew P. Hayward 1, Anthony Tordillos 1, Christopher Fragoso 1, Maria A. Moreno 1, Joe Tohme 2, Albert P. Kausch 3, John P. Mottinger 3, Stephen L. Dellaporta 1,*
The maize Mutator (Mu ) is one of the most aggressively mobile transposon families yet characterized in any organism. Mutator lines were first described as a genetic system that increased the mutation rate by 30-fold . The "Mutator" trait did not segregate according to a simple one-gene model, with nearly 100% of progeny between crosses of Mutator lines and non-Mutator lines exhibiting high mutation rates . The first mutant allele to be cloned and characterized from a Mutator line contained a 1.4 kb insertion (later named Mu1 ) in the alcohol dehydrogenase (adh1 ) gene of maize . The Mu1 element had ~215 bp highly homologous terminal inverted repeats (TIRs), and was flanked by a 9 bp target sequence duplication at its site of insertion . When Robertson's Mu lines were examined by Southern hybridization, plants were shown to possess between 10-70 copies of Mu -related sequences . These early studies suggested that active transposable elements were the genetic basis for the high mutation rates found in Robertson's Mutator lines.
Continued characterization of Mu lines has elucidated a family of transposons that is both diverse and complex. More than a dozen different Mu elements are currently known (reviewed in ), including the autonomous MuDR element that encodes for an active transposase, MURA , . All active (mobile) Mu elements share highly similar TIRs but their internal regions can vary considerably in both size and sequence [reviewed in , -]. MURA binds to a highly conserved region within the Mu TIR, promoting transposition both in cis and in trans for other Mu elements found in the plant genome . In this way, the presence and activity of MuDR determines whether all non-autonomous Mu elements are mobile or immobile in the plant's genome. Other epigenetic factors, such as the cytosine methylation state of Mu elements, affect mobility of individual elements (reviewed in ).
A number of factors account for the exceptionally high mutation rates in Mu lines. First, active Mu lines can contain over 100 Mu elements per genome (reviewed in ), a number that can be maintained from generation to generation . On average, each transposon is responsible for one new insertion event every generation, either non-conservatively through transposition or conservatively through duplication . Studies show that Mu elements prefer to insert into low copy number (non-repetitive) DNA , and genome-wide analysis suggests that Mu elements preferentially insert into regions of the genome that are transcribed . For instance, analysis of the RescueMu element (described below) shows that 69% of its insertion sites are located in putatively expressed genomic sequences  even though the maize genome appears to be made up of more than 80% repetitive sequences . Finally, Mu elements exhibit little target site sequence specificity, allowing for essentially uniform genome-wide mutagenesis (reviewed in )....