Identification of an Active New Mutator Transposable Element in Maize

Robertson's Mutator (Mu) system has been used in large scale mutagenesis in maize, exploiting its high mutation frequency, controllability, preferential insertion in genes, and independence of donor location. Eight Mutator elements have been fully characterized (Mu1, Mu2 /Mu1.7, Mu3, Mu4, Mu5, Mu6/7, Mu8, MuDR), and three are defined by TIR (Mu10, Mu11 and Mu12). The genome sequencing revealed a complex family of Mu-like-elements (MULEs) in the B73 genome. In this article, we report the identification of a new Mu element, named Mu13. Mu13 showed typical Mu characteristics by having a ∼220 bp TIR, creating a 9 bp target site duplication upon insertion, yet the internal sequence is completely different from previously identified Mu elements. Mu13 is not present in the B73 genome or a Zea mays subsp. parviglumis accession, but in W22 and several inbreds that found the Robertson's Mutator line. Analysis of mutants isolated from the UniformMu mutagenic population indicated that the Mu13 element is active in transposition. Two novel insertions were found in expressed genes. To test other unknown Mu elements, we selected six new Mu elements from the B73 genome. Southern analysis indicated that most of these elements were present in the UniformMu lines. From these results, we conclude that Mu13 is a new and active Mu element that significantly contributed to the mutagenesis in the UniformMu population. The Robertson's Mutator line may harbor other unknown active Mu elements.