WHAT DETERMINES THE FOLDING OF THE CHROMATIN FIBER

In this review, we attempt to summarize, in a critical manner, what is currently known about the processes of condensation and decondensatio n of chromatin fibers, We begin with a critical analysis of the possib le mechanisms for condensation, considering both old and new evidence as to whether the linker DNA between nucleosomes bends or remains stra ight in the condensed structure, Concluding that the preponderance of evidence is for straight linkers, we ask what other fundamental proces s might allow condensation, and argue that there is evidence for linke r histone-induced contraction of the internucleosome angle, as salt co ncentration is raised toward physiological levels, We also ask how cer tain specific regions of chromatin can become decondensed, even at phy siological salt concentration, to allow transcription, We consider lin ker histone depletion and acetylation of the core histone tails, as po ssible mechanisms, On the basis of recent evidence, we suggest a unifi ed model linking targeted acetylation of specific genomic regions to l inker histone depletion, with unfolding of the condensed fiber as a co nsequence.