The discrepancy between chromatin factor location and effect

The influence of chromatin on many cellular processes is well appreciated. Much has been learned by studying the role of chromatin remodeling and modifying complexes on individual genes. The seemingly straightforward models that inevitably arise from such studies are challenged by genome-wide analyses. Two recent studies in Saccharomyces cerevisiae provide unprecedented coverage of both the genome-wide location and the effect on gene expression for the majority of chromatin factors. Comparison of the overlap between location and expression effects reveals a large disconnect, with on average only 2.5% of occupied genes showing changes in expression. It is also interesting that only 24% of all expression effects are associated with chromatin factor occupancy. The large difference between location and effect likely reflects general properties inherent to regulation of gene expression through chromatin in yeast. Explanations for the discrepancy include gene-specific properties that exert a requirement for certain factors only on specific genes, as well as functional redundancy, whereby loss of a particular factor is compensated by others that function in a distinct but nevertheless compensatory manner. Since the majority of chromatin factor perturbations do show significant effects on specific subsets of genes, this implies the presence of different types of gene-specific properties that determine which chromatin factors a particular gene requires for proper expression. Understanding these gene-specific properties should be the focus of future studies aimed at understanding regulation of gene expression through chromatin.