The MuDR transposon terminal inverted repeat contains a complex plant promoter directing distinct somatic and germinal programs

The Mu transposons of maize are under stringent developmental control. Elem ents excise at high frequencies in terminally dividing somatic cells, but n ot in meristems. Mu elements in germinal cells amplify, without excision, a nd insert throughout the genome. All activities require MuDR, which encodes two genes, mudrA and mudrB, whose near-identical promoters are located in the transposon terminal inverted repeats (TIR). We have fused the 216 bp TI R of the mudrB gene to GUS and luciferase reporters. We demonstrate that TI RB programs reporter expression in diverse, meristematic somatic cells, par adoxically in those cells in which Mu excisions are repressed. In germinal cells, immature tassel and mature pollen, reporter expression increases up to 20-fold compared to leaf. By RNA blot hybridization, we demonstrate that endogenous mudrB and mudrA transcripts increase significantly in mature po llen; sequence comparisons demonstrate that the MuDR TIRs contain plant cel l-cycle enhancer motifs and functionally defined pollen enhancers. Therefor e, the MuDR TIR promoters are developmentally regulated in both somatic and germinal tissues. Because database sequence analysis suggests that the MuD R TIR enhancers should be functional in both monocots and dicots, we sugges t that the native MuDR promoter be used in attempts to transfer the unique behavior of Mu transposition to heterologous hosts.