TISSUE-SPECIFIC DNASE-I-SENSITIVE SITES OF THE MAIZE P-GENE AND THEIRCHANGES UPON EPIMUTATION

A map has been developed of nuclease-hypersensitive sites of P-rr, the standard allele of the P-locus of Zea mays L. Using a traditional DNa se I assay, eight such sites have been found that are specific for the expressing tissue and span a region of more than 25 kb of the P-locus , making it one of the largest plant genes yet described. The maps of the standard allele have also been compared with the recently describe d moderately stable P-pr allele, which arose from epimutation. Six of the eight sites exhibit the same tissue-specificity in P-pr plants, wh ile two stay repressed as in non-expressing tissues of plants with the standard allele. Interestingly, the two repressed sites coincide with two hypermethylated restriction sites that have previously been corre lated with the expression potential of the P-pr allele. On the other h and, four of the DNase I sites, coinciding with CpG islands that were not hypermethylated by the epimutation, also showed no differences in their sensitivity to DNase I between the standard allele and the P-pr allele. This suggests that the epimutation affects both site-specific methylation changes and a specific local chromatin structure of the P gene involved in its regulation.