Regulation of the chicken lysozyme locus in transgenic mice.

The chicken lysozyme locus is transcriptionally activated during macrophage differentiation. Each cis-regulatory element has its unique activation stage during cell differentiation, whereby maximal transcriptional activity of the gene is only observed when all cis-elements are active. The complete chicken lysozyme locus is expressed position independently and at a high level in macrophages of transgenic mice. For correct transgene regulation, the cooperation of all cis-regulatory elements is required. These cis-regulatory elements specify the mode of regulation and we observe the same expression pattern of the transgene in the mouse and the endogenous gene in chicken macrophages. This indicates that the transcription factors responsible for chicken lysozyme regulation are highly conserved in evolution. The endogenous mouse lysozyme gene is regulated differently. The chromatin of the lysozyme locus is highly structured in the transcriptionally active, as well as in the inactive state. The transcriptional activation of the lysozyme locus is accompanied by extensive chromatin rearrangements, which are disturbed when one essential cis-regulatory element is deleted and the transgenes are subjects to genomic position effects. Based on these results, we propose that a distinct chromatin architecture of a gene locus is required for its correct activation.