ABSTRACT Mol Cell Biol 2, 211-220.

We have probed the accessibility of the genes for rRNA in Physarum polycephalum bv using the phot’oreac’ive DNA cross-Iinking agent 4.5Õ .8-trimethyl psoraien. Nuclei isolated from actively growing Physarum were treated with trimethyl psoralen and irradiated wi’h 360-nm light in order to form cross-links. The palindromic, extrachromosomal rDNA then was isolated. and the positions of cross-links were determined by electron microscopy of the DNA under totaIly denaturing condi’ions. The results indicate that the frequency of cross-linking, after correction for base sequence bias of the reaction, is up to sixfold higher in the transcribed regions than in the central or the terminal spacer regions. There is no detec’able liclerogenei’y among ihe different rDNA molecules or be’ween the halves of a single molecule. Cross-linked molecules invariably occur in a linear as opposed to cruciform structure. The preferential cross-linking of the transcribed region was nearly eliminated in spherules, a dormant transcriptionally Inactive form in the Physarum life cycle.