Genomic position effects lead to an inefficient reorganization of nucleosomes in the 5'-regulatory region of the chicken lysozyme locus in transgenic mice.

The chicken lysozyme locus is gradually activated during macrophage development exhibiting a specific chromatin structure with each differentiation state. Its small size and the extensive characterization of its cis-regulatory elements allows us to study even subtle changes in chromatin structure of the entire gene locus during transcriptional activation. Tissue-specific and position independent expression of the lysozyme locus in transgenic mice requires the cooperation of all cis-regulatory elements. In order to elucidate further the molecular basis of locus activation, we have determined nucleosome positions within the complete 5'-regulatory region of the chicken lysozyme locus in chicken myeloid cell lines and transgenic mice. Each cis-regulatory element develops its unique nucleosomal structure and each one remodels chromatin differently. The nucleosomal organization of the endogenous gene in chicken cell lines and the transgene in the mouse turned out to be identical, enabling us to study the influence of cis-regulatory deletions on the development of an active chromatin structure in transgenic mice. Transgenes with a deletion of an important cis-regulatory element show an impediment in nucleosome reorganization as compared with the complete lysozyme locus. We demonstrate that multicopy transgene-clusters in position dependently expressing mouse lines exhibit a heterogeneous chromatin organization.