The -3.9 kb DNaseI hypersensitive site of the chicken lysozyme locus harbours an enhancer with unusual chromatin reorganizing activity.

Tissue specific regulation of the chicken lysozyme locus is achieved by a combination of positive and negative cis-regulatory elements. Here we describe the molecular characterization of a newly discovered enhancer element located -3.9kb upstream of the transcription start. The -3.9kb enhancer is activated early in macrophage differentiation, as indicated by chromatin reorganization in macrophage precursor cells. Interestingly, enhancer activation leads to nucleosome phasing. Tissue specificity of expression is achieved by a combination of 5'-sequences with ubiquitous enhancer activity and 3'-flanking sequences. The 5'-half contains binding sites for members of the nuclear factor I transcription family and a yet unknown protein. We could show by in vivo footprinting that the ubiquitously expressed factors occupy their binding sites only in lysozyme expressing cells. We conclude that a specific chromatin architecture may be responsible for the differential activity of the enhancer.