9603638 Chandler Technical The b regulatory gene of maize encodes a basic-helix-loop-helix protein, which together with other regulatory proteins, activates the transcription of genes encoding the anthocyanin biosynthetic enzymes. Different b alleles activate the pathway in distinct tissues and at different developmental times. Because the alleles are differentially expressed and the amount of pigment produced is a sensitive indicator of regulatory protein levels, such expression can be studied at the plant level. It has been found that specific alleles, when in the same genome with other alleles can alter the second allele in a stable fashion and such alteration cannot be explained by Mendelian concepts. The name given to such phenomena is paramutation. Previous studies have shown that the sequences required for paramutation of b are tightly linked to the locus and lie in the 5' flanking region. However, DNA sequencing and extensive restriction mapping have not identified structural differences between the b alleles undergoing paramutation. Two models are proposed and experiments are devised to test the models. One model is that of unidirectional gene conversion which is consistent with the stability of the altered allele. The other model suggests that changes in chromatin structure may cause the heritable alterations in transcription that occur during paramutation. Questions to be asked are: What cis-acting sequences and other genes participate in paramutation? What cis-acting sites are required for b transcription? Do chromatin, DNA methylation or structural differences correlate with paramutation? Are b sequences in ectopic locations able to participate in paramutation? A variety of mutagenesis strategies are proposed to identify cis-acting sites and trans-acting loci required for the establishment or maintenance of paramutation and for b transcription. Non-technical Transcription can be altered by chromosome position or exposure to alternate alleles, suggesting that genes are not regulated just by local cis-acting sequences and factors that interact with those sequences. Examples of chromatin environments regulating gene expression come from studies of mammalian X-chromosome inactivation, genome imprinting, silencing of yeast mating-type loci, position effect variegation in Drosophila, paramutation and gene silencing in plants. Mechanisms are not understood in any system, although evidence suggests that either DNA or chromatin modification, or both, could be involved. The proposed studies to probe allelic interactions that alter transcription at a corn regulatory gene should help to address this deficiency.
