Light-regulated changes in DNase I hypersensitive sites in the rRNA genes of Pisum sativum

We have examined the rDNA chromatin of Pisum sativum plants grown with or without exposure to light for the presence of DNase I hypersensitive sites and possible developmental changes in their distribution. Isolated nuclei from pea seedlings were incubated with various concentrations of DNase I. To visualize the hypersensitive sites, DNA purified from these nuclei was restricted and analyzed by gel blot hybridization. We find that several sites exist in both the coding and noncoding regions of rDNA repeating units. Several of the sites in the nontranscribed spacer region are present in the light but are absent in the dark. Conversely, the hypersensitive sites within the mature rRNA coding regions are present in the dark but absent in the light. There are two major length variants of the rRNA genes in P. sativum var. Alaska. The sites in the nontranscribed spacer region that appear during the light treatment occur only in the shorter of these two length variants in this cultivar.