Gene expression and chromatin structure in the cellular slime mold, Dictyostelium discoideum

Micrococcal nuclease digestion of chromatin from growing cells reveals a structural organization which differs for genes transcribed at diverse rates. The late cAMP dependent prespore genes which are not transcribed in growing cells are found in growing cells in a regular nucleosomal repeat with an average spacing of 168 nucleotides. By contrast genes expressed at a low level in growing cells show an irregular pattern of bands with an average distance between bands of 80 nucleotides. The sizes of the bands generated from the transcribed genes are consistent with the concept that transcription results in the loss of the linker region histone H1 with concomitant sliding of nucleosomes to generate close packed ("slipped") di, tri, and tetra nucleosomes lacking the linker region. Further analysis of dinucleosomes released by micrococcal nuclease digestion reveals that transcriptionally active genes are found associated with dinucleosomes species which may be lacking histone H1. The length of DNA protected by these dinucleosomes is heterogeneous, ranging from 250 to 300 nucleotides. Methodology is described which has been adapted to allow two dimensional hybridization mapping of nucleoprotein complexes on single copy Dictyostelium genes.