Chromatin fine structure profiles for a developmentally regulated gene: reorganization of the lysozyme locus before trans-activator binding and gene expression

The chicken lysozyme locus is activated in a stepwise fashion during myeloi d differentiation. We have used this locus as a model to study at high reso lution changes in chromatin structure both in chicken cell lines representi ng various stages of macrophage differentiation and in primary cells from t ransgenic mice. In this study we have addressed the question of whether chr omatin rearrangements can be detected in myeloid precursor cells at a stage well before overt transcription of the lysozyme gene begins. In addition t o restriction enzyme accessibility assays and DMS footprinting, we have app lied new, very sensitive techniques to assay for chromatin changes. Particu larly informative was UV photofootprinting, using terminal transferase-depe ndent PCR and nonradioactive detection. We find that the basic chromatin st ructure in lysozyme nonexpressing hematopoietic precursor cells is highly s imilar to the pattern found in fully differentiated lysozyme-expressing cel ls. In addition, we find that only in nonexpressing cells are dimethylsulfa te footprints and UV photofootprints affected by trichostatin, an inhibitor of histone deacetylation. These results are interpreted to mean that most chromatin pattern formation is complete before the binding of end-stage tra ns-activators, supporting the notion that heritable chromatin structure is central to the stable epigenetic programs that guide development.