Abstract
The mouse M-lysozyme gene is exclusively expressed in myeloid cells of the blood system being progressively turned on upon cell differentiation. In this study the mechanism controlling this tissue- and differentiation stage-specific gene expression was analyzed at the level of chromatin structure. A complex pattern consisting of constitutive and differentiation dependent DNasel hypersensitive sites (HSs) was found in a set of various myeloid cell lines, representing different stages of maturity. The chromatin of a lymphoid cell line, which does not express the lysozyme gene, is completely insensitive to DNasel digestion. Chromatin analysis of two multipotent hematopoietic stem cell lines which can be differentiated in vitro to mature myeloid cells confirmed that these identified DNasel HSs are specific for distinct differentiation stages, rather than being a characteristic feature of the cell lines. Additionally, the stem cell studies revealed that the hypersensitivity of the chromatin domain is already established at the multipotent stage. DNA fragments spanning a cell type- and differentiation stage-specific cluster of HSs in the 3' region of the gene showed enhancer activity in all cell types tested. In the light of this lack of specificity, we suggest that cell type-specific modification of the chromatin structure in this region may play a role in determining the binding of a widespread transcription factor, and hence contribute to the time specificity of lysozyme M gene expression.