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 c ombination of positive and negative cis-regulatory elements. Here we descri be the molecular characterization of a newly discovered enhancer element lo cated -3.9 kb upstream of the transcription start. The -3.9 kb enhancer is activated early in macrophage differentiation, as indicated by chromatin re organization in macrophage precursor cells. Interestingly, enhancer activat ion leads to nucleosome phasing. Tissue specificity of expression is achiev ed 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 act ivity of the enhancer. (C) 1999 Elsevier Science B.V. All rights reserved.