Nucleosome Organisation of the β-Lactoglobulin Gene

Abstract

In most mammals, the major whey protein beta-lactoglobulin (βlg) represents a marker for tissue-specific, temporally regulated gene expression in the mammary gland. Prolactin, the major lactogenic stimulus which activates βlg expression acts through a cytoplasmic signalling cascade ending in the activation of the transcription factor STAT5. Although much is known about the complexities of this signalling cascade, little is known about how this transcription factor functions within the context of chromatin. Using DNaseI as a probe of chromatin structure we have identified temporally regulated elements within the ovine βlg gene domain. The appearance of these hypersensitive sites accompanies changes in expression state of the βlg gene. Changes in DNaseI hypersensitivity at the proximal promoter region, while reflecting STAT5 activation, is not dependent upon STAT5 interaction at this site. We have mapped the nucleosome positions over the entire βlg gene, both in vitro using the monomer extension assay and in vivo using cuprous phenanthroline to probe for nucleosome-linker positions. The specific positioning pattern detected, which reflects strong sequence-directed positioning over the proximal promoter, complement the STAT5 consensus sites within this region. The comparison of both the functional and chromatin data enables a model for blg gene transcription to be developed.