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Epigenetic Silencing and Activation of a Maize r Gene

Jerry L. Kermicle


The r Gene Family
The r (red color) family of maize genes codes for transcriptional activators of the basic helix-loop-helix class (Ludwig et al. 1989) that regulate genes in the anthocyanin biosynthetic pathway (for review, see Dooner et al. 1991). Members of the r gene family, which also includes b (booster) and lc/sn (leaf color and scutellar node color), show high sequence homology in their coding regions, and their products are functionally interchangeable (Goff et al. 1990; Ludwig et al. 1990). However, sequences in their control regions are divergent, and these differences govern the distribution of anthocyanin pigmentation plant part by plant part (Styles et al. 1973; Radicella et al. 1992). Because the pigment is conspicuous, subtle differences between genes and alleles have been discerned. Genes of the r family thus serve as sensitive reporters of their own regulation. Perhaps for this reason, a number of epigenetic phenomena involving this gene family have been discovered (for review, see Brink 1973; Patterson and Chandler 1995). Those involving the r locus are discussed here; those involving b are discussed separately (Chandler et al., this volume).

Among strains of maize collected from diverse geographic sources, the r locus on chromosome 10 sometimes is represented by one r gene, sometimes by two or more, thus comprising a gene complex. When multiple, each r gene constitutes a separate complementation group, with different combinations of r genes resulting in different pigmentation patterns. For example, the R-r:standard complex comprises a simple r gene, P, which confers...

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