Transposon-mediated chromosomal rearrangements and gene duplications in the formation of the maize R-r complex.

R-r controls the production of anthocyanin pigment in plant parts and the aleurone layer of seeds through the production of a family of related transcriptional activating proteins of the helix-loop-helix type. The R-r complex comprises a series of repeated, homologous components arranged in both direct and inverted orientations. These include the P component, a simple R gene that confers pigmentation of plant parts, and the S subcomplex that consists of a truncated inactive R gene called q, and two functional R genes, S1 and S2, that pigment the aleurone. The S genes are arranged in an unusual inverted head-to-head orientation. The identity of each functional component was confirmed by microprojectile bombardment of intact maize tissues with cloned genomic DNA and by analysis of in vivo mRNA populations. Sequence analysis suggests that the S subcomplex was derived through the rearrangement of a simple P-like progenitor element. At the rearrangement breakpoints, features typical of the CACTA family of transposable elements were found. The location and arrangement of these CACTA element sequences implies that this element may have mediated the chromosomal rearrangements that led to the formation of the R-r complex. The unusual structure of R-r explains much of the meiotic instability of the complex.