Allele in Maize

The identification of the autonomous or transposase-encoding element of the Mutator (Mu) transposable element system of maize is necessary to the characterization of the system. We reported previously that a transcript homologous to the internal region of the MuA element is associated with activity of the Mutator system. We describe here the cloning of another Mu element, designated MuA2, that cosegregates with Mutator activity as assayed by somatic instability of the al-Mum2 allele. The MuA2 element has features typical of the transposable elements of the Mutator family, including the 2 1 O-bp terminal inverted repeats. Several lines of evidence suggest that MuA2 is an autonomous or transposase-encoding element of the Mu family: (1) MuA2 cosegregates with a genetically defined element that regulates somatic mutability of the al-Mum2 allele; (2) MuA2 is hypomethylated while most other MuA2-hybridizing sequences in the genome are extensively methylated; (3) the increase of the copy number of MuA2 is concomitant with the increase of regulator elements; (4) MuA2-like elements are found in Mutator lines but not in non-Mutator inbreds. We propose that autonomous or transposase-encoding elements of the Mu family may be structurally conserved and MuA2-like. T RANSPOSABLE element systems in maize are usually composed of two components: the autonomous element and nonautonomous elements. An autonomous element encodes the functions necessary for its own transposition and can also act in trans to promote the transposition of nonautonomous elements. In the Ac-Ds and Sprn (En) systems, the autonomous element for each system is a single, structurally conserved element whereas nonautonmous elements are structurally heterogeneous. The Ds elements of the Ac-Ds system vary in structure, ranging from sequences derived from Ac by deletions to those that are virtually unrelated to the Ac element, except for the terminal inverted repeats (TIRs). Defective Sprn (dSpm) elements of the Spm system are usually deletion-derivatives of the Sprn element (reviewed by