Transcriptionally active MuDR, the regulatory element of the mutator transposable element family of Zea mays, is present in some accessions of the Mexican land race Zapalote chico.

To date, mobile Mu transposons and their autonomous regulator MuDR have been found only in the two known Mutator lines of maize and their immediate descendants. To gain insight into the origin, organization, and regulation of Mutator elements, we surveyed exotic maize and related species for cross-hybridization to MuDR. Some accessions of the mexican land race Zapalote chico contain one to several copies of full-length, unmethylated, and transcriptionally active MuDR-like elements plus non-autonomous Mu elements. The sequenced 5.0-kb MuDR-Zc element is 94.6% identical to MuDR, with only 20 amino acid changes in the 93-kD predicted protein encoded by mudrA and ten amino acid changes in the 23-kD predicted protein of mudrB. The terminal inverted repeat (TIR) A of MuDR-Zc is identical to standard MuDR; TIRB is 11.2% divergent from TIRA. In Zapalote chico, mudrA transcripts are very rare, while mudrB transcripts are as abundant as in Mutator lines with a few copies of MuDR. Zapalote chico lines with MuDR-like elements can trans-activate reporter alleles in inactive Mutator backgrounds; they match the characteristic increased forward mutation frequency of standard Mutator lines, but only after outcrossing to another line. Zapalote chico accessions that lack MuDR-like elements and the single copy MuDR a1-mum2 line produce few mutations. New mutants recovered from Zapalote chico are somatically stable.