The activation of the chicken lysozyme locus is a cooperative process.

The chicken lysozyme gene is a marker for the myelomonocytic lineage of the hematopoietic
system. In early experiments we demonstrated that correct activation of the chicken lysozyme locus
in macrophages of transgenic mice requires the complete set of cis-regulatory elements. Different
cis-elements are activated at distinct developmental stages and their chromatin structure i s
differentially remodelled. We have shown that the early onset of transcriptional activation of the
chicken lysozyme locus is entirely dependent on enhancer elements which are structurally activated
early in development (-6.1 kb and -3.9 kb early enhancers). However, the structural reorganization
of the early enhancers requires the presence of promoter sequences. We concluded from these
experiments that the early enhancers and the promoter cooperate in order to activate the lysozyme
locus. Subsequently, we performed experiments aimed at elucidating the cis-regulatory
requirements of chromatin rearrangement at the early enhancers. The -6.1 kb enhancer is well
characterized at the molecular level and all transcription factors contributing to its activity in
transfection studies are known. We have placed this element into a new sequence context on the
lysozyme locus by deleting extended flanking regions and analyzed this construct in transgenic
mice. Surprisingly, its chromatin rearrangement ability as judged from DNaseI hypersensitive site
formation was impaired. We conclude from this experiment that the cooperation of enhancer core
and flanking sequences is necessary for enhancer activity. We hypothesize that all sequences of a
gene locus serve a purpose in the developmental control of its activation.