The rice R gene family: two distinct subfamilies containing several miniature inverted-repeat transposable elements

The R and B genes of maize regulate the anthocyanin biosynthetic pathway an d constitute a small gene family whose evolution has been shaped by polyplo idization and transposable element activity. To compare the evolution of re gulatory genes in the distinct but related genomes of rice and maize, we pr eviously isolated two R homologues from rice (Oryza sativa). The Ra1 gene o n chromosome 4 can activate the anthocyanin pathway, whereas the Rb gene, o f undetermined function, maps to chromosome 1. In this study, rice R genes have been further characterized. First, we found that an Rb cDNA can induce pigmentation in maize suspension cells. Second, another rice R homologue ( Ra2) was identified that is more closely related to Ra1 than to Rb. Domesti cated rice and its wild relatives harbor multiple Ra-like and Rb-like genes despite the fact that rice is a true diploid with the smallest genome of a ll the grass species analyzed to date. Finally, several miniature inverted- repeat transposable elements (MITEs) were found in R family members. Their possible role in hastening the divergence of R genes is discussed.