Regulation of the GAL structural genes in the yeast Saccharomyces cerevisiae is implemented by the products of GAL-specific (GAL4, GAL80, GAL3) and general (GAL11, SWI1, 2, 3, SNF5, 6, numerous glucose repression) genes. Recent work has 1) yielded significant new insights on the DNA binding and transcription activation/Gal80 protein binding functions of the Gal4 activator protein, 2) described the characterization of purified Gal4 protein-Gal80 protein complexes, 3) deconvoluted the multiple and complex glucose repression pathways acting on GAL genes, 4) suggested a new mechanism for the Gal3 protein-mediated induction of GAL structural gene expression, 5) introduced Gal1 protein, a structural gene product, into the regulation scheme, and 6) extended our already substantial understanding of GAL regulatory gene control. The mechanisms which control structural and regulatory gene expression in the GAL family are compared and GAL structural/regulatory gene chromatin structure is discussed.

For a couple of decades the chromatin field has endured undeserved neglect. Indeed, what could be so exciting about a monotonous repeating structure whose purpose in life was to package DNA? Chromatin glamour is triumphantly back, due to the realization that chromatin is a major player in the regulation of gene expression and other nuclear processes that occur on the DNA template. The dynamics of the structure that regulates transcription is itself regulated by a variety of complex processes, including histone postsynthetic modifications, chromatin remodeling, and the use of nonallelic histone variants. This review is an attempt to understand the mechanisms of action of the evolutionarily conserved variant H2A.Z, a player with a variety of seemingly unrelated, even contrary, functions. This attempt was prompted by the recent avalanche of genome-wide studies that provide insights that were unthinkable until very recently.

The importance of forces in biology has been recognized for quite a while but only in the past decade have we acquired instrumentation and methodology to directly measure interactive forces at the level of single biological macromolecules and/or their complexes. This review focuses on force measurements performed with the atomic force microscope. A general introduction to the principle of action is followed by review of the types of interactions being studied, describing the main results and discussing the biological implications.

Unfixed chicken erythrocyte chromatin fibers in very low salt have been imaged with a nning force microscope operating in the tapping mode in air at ambient humidity. These images reveal a threedimensional organizaffon of the fibers. The planar "6zig-zag" conformation is rare, and extended "beads-on-a-string" fibers are seen only in chromatin depleted of histes Hi and H5. Glutaraldehyde fixation reveals very similar structures. Fibers fixed in 10 mM salt appear somewhat more compacted. These results, when compared with modeing stude, suggest that chromatin fibers may exist as frregular three-dimensional arrays of nucleosomes even at low ionic strength.The structure of the chromatin fiber in low salt concentrations remains controversial. Electron microscopy (EM) experiments, most of which utilized the Miller spreading technique (1), typically showed extended "beads-on-a-string" or "open zig-zag" structures (refs. 2 and 3; for reviews, see refs. 4-6). At slightly higher ionic strength (-10 mM NaCI), somewhat more compact, "closed" zig-zags of nucleosomes were observed (7-9). Only upon further addition of NaCl to about 100 mM did these extended structures condense to form the so-called 30-nm fiber (8), which resembles structures observed in situ (7, 9, 10). However, there has been concern that strong interactions of the fiber with the EM support surface, and the dehydration produced by the high vacuum conditions, could distort the structure, especially at low ionic strength.Attempts to circumvent these problems used solution studies. The first scattering experiments suggested that the nucleosomes were densely packed in a linear array (11,12 To address this controversy, a study was performed using the three-dimensional imagi capabilities ofa scanning force microscope, which makes it possible to image chromatin fibers under less damaging conditions (22,23). The samples are never vacuum dried and are scanned in air at about 50%o relative humidity. Under these conditions a film of liquid water resides on the support surface (24). The newly developed tapping operation mode (25, 26) was employed, in which a stiff cantilever is oscillated near its resonance frequency with amplitudes typically in the range of 10-20 nm as the sample is scanned laterally. The oscillation amplitude is kept constant via feedback control. This operation has several advantages over the contact mode, in which a tip is pulled across the sample. Tip-sample forces are lighter than in the contact mode. Moreover, since most of the force is perpendicular to the surface, the sample experiences minimal lateral deformation during scanning, thus improving spatial resolution (25, 26). MATERIAL AND METHODSPreparation and Fization of Chromatin. Chicken erythrocyte chromatin was prepared essentially as described (27), with a reduction in the amount of micrococcal nuclease to allow isolation of long fibers (28). Soluble chromatin was dialyzed versus 5 mM triethanolamine/HC1 (pH 7.0), with or without 10 mM NaCl and was stored on ice. In a few experiments...