PARAMUTATION, AN ALLELIC INTERACTION, IS ASSOCIATED WITH A STABLE ANDHERITABLE REDUCTION OF TRANSCRIPTION OF THE MAIZE-B REGULATORY GENE

The b gene of maize encodes a transcriptional activator of anthocyanin pigment biosynthetic genes. Certain b alleles undergo paramutation: a unidirectional, heritable alteration of one allele caused by the pres ence of another allele. B-I (intensely pigmented plant) is always chan ged to B' (weakly pigmented plant) in the B'/B-I heterozygote, such th at all progeny receive the B' allele. The ''new B', which was B-I in t he previous generation, is weakly pigmented and fully capable of chang ing another B-I allele into B'. It was not previously known whether pa ramutation is associated with altered b expression, altered B protein function or both. Our results show that B' acts in trans to suppress t he transcription of B-I, with transcription remaining low in subsequen t generations, even when the original B' allele segregates away. The p roducts of B-I and B' are equally capable of activating the transcript ion of their target genes, indicating they are functionally equivalent . Genomic restriction maps, DNA sequence and methylation of B' and B-I were compared. Despite dramatic differences in phenotype and transcri ption of B' and B-I, no evidence for rearrangements, changes in sequen ce or changes in methylation was found. These results provide no suppo rt for models involving ''dominant negative'' proteins, gene conversio n or transposable element interactions. We suggest that b paramutation involves a physical interaction between the alleles that suppresses t ranscription and promotes a change in chromatin structure that is heri table.