Mu Transposons

The Mutator system

Mutator (Mu) elements are a Class II family of transposons in maize. These elements can duplicate rapidly in the maize genome, and in doing so, can cause a very high mutation rate (hence the name). Mu elements in maize are regulated by the MuDR class of elements. These autonomous elements encode two genes, mudrA, the transposase, and mudrB, a helper gene. Although transposons carrying homologs of mudrA are found in a wide variety of organisms, mudrB is found only in maize and its closes relatives. In the presence of active MuDR elements, non-autonomous elements, which share only pairs of terminal inverted repeats with MuDR, will transpose at frequencies approaching 100% (one duplication per element per generation). Because of this property, and because they prefer to hop into genes, Mu elements have been used to isolate a large number of genes in maize. In addition to its utility, the Mutator system has helped to illuminate a variety of phenomena, from horizontal transfer to the causes and consequences of epigenetic variation. Mutator behavior is extremely dynamic. In addition to moving about within (and sometimes between) genomes, Mu elements are prone to change; deletions, transpositions and shifts between active and silent states occur at high frequencies. Further, Mu activity is easily scorable as changes in somatic excision activity, spots on corn kernels. Like a bacterial culture, the system evolves at a rate that we can watch; radical changes in behavior can occur within a single generation. Using classical genetics, those changes can be isolated and amplified for further study. We employ a minimal version of the Mutator system to tease out and isolate specific sources of variation. A highly inbred maize lineage is maintained that carries a single MuDR element and a single non-autonomous element inserted into a color gene. Because it is relatively simple and its behavior is highly reproducible, this system allows us tease out new sources of variation in its behavior and regulation. The effects of factors such a position, copy number and the tendency of the element to rearrange itself can be easily examined. The Mutator system provides us with a perfect laboratory for investigating the dynamics of transposon behavior in vivo. To compliment the genetic analysis, genomic analysis of Mu transposon sequences in a wide variety of plants, fungi, and animals gives us a window into the evolution of the widely distributed MULE (Mu-like element) family of elements.