Epigenetic Silencing of Mu Transposable Elements in Maize

Abstract

Transposable genetic elements interact with specific DNA-binding proteins to facilitate their excision from the chromosome and their integration elsewhere. Autonomous transposons carry genes that encode some of these proteins, and so provide the trans-acting functions, such as transposase, required for their own transposition and that of related non-autonomous elements. Transposons are frequently subject to epigenetic regulation and share many of the properties associated with chromosomal genes that are epigenetically regulated: They are found in multiple copies; they are integrated at diverse sites and are often subject to position effects; they sometimes encode multivalent DNA-binding proteins that can alter chromatin structure; and they are often subject to DNA methylation. It is possible that some epigenetic mechanisms of gene regulation evolved as a means of controlling transposons and viruses that were otherwise capable of widespread integration throughout the genome (for review, see Martienssen and Richards 1995).

Epigenetic regulation of transposons can be monitored in vivo because the expression of chromosomal genes disrupted by transposon insertion is influenced by transposase in two different ways. (1) Transposon excision depends on transposase and often restores wild-type chromosomal gene expression. (2) Transposons inserted in regulatory regions of some genes interact with transposase, resulting in alterations in chromosomal gene expression without transposon excision (McClintock 1965b; Peterson 1966; Rutledge et al. 1988; Boeke 1989; Robertson and Engels 1989; Gierl 1990; Corces and Geyer 1991). These regulatory interactions are particularly convenient assays for transposon activity, because they affect gene expression in a uniform fashion without relying on occasional transposon...