Chromatin structure and the regulation of gene expression: remodeling at the MMTV promoter

The role of the various levels of chromatin organization in the control of eukaryotic gene expression is discussed on the basis of recent advances in our understanding of chromatin structure in well-defined model systems. Particular attention is devoted to the precise structure and the possible functions of positioned nucleosomes and to the enzymatic mechanism of nucleosome remodeling. Some of the principles involved are illustrated with genomic footprinting results obtained with the mouse mammary tumor virus promoter, the nucleosomal organization of which is remodeled following transcriptional induction by steroid hormones. In this system a positioned nucleosome is responsible for transcriptional repression prior to hormone administration and participates in hormonal induction by facilitating the functional interaction among transcription factors on the promoter.