Disruption of the typical chromatin structure in a 2500 base-pair region at the 5′ end of the actively transcribed ovalbumin gene.

Abstract

We examined the chromatin organization of -3 kb of DNA in the 5' -end flanking region of the ovalbumin gene in chicken erythrocyte and oviduct cell nuclei. With specific DNA probes and an indirect end-labeling technique, we analysed the pattern of the DNA fragments obtained after micrococcal nuclease digestion and generated comparative maps of the nuclease cuts. This region of the chicken genome displays a "typical" chromatin arrangement in erythrocyte nuclei, with nucleosomes apparently positioned at random. In contrast, in oviduct nuclei, the same region has an "altered" chromatin structure, and lacks a typical nucleosomal array. The existence of specifically positioned proteins and of alterations in the DNA secondary structure in this region of the oviduct chromatin is suggested by comparison of the nuclease cleavage maps which reveals specific changes: disappearance of nuclease cuts present in "naked" and erythrocyte chromatin DNAs, and appearance of new cuts absent from these DNAs.

Signals in chicken beta-globin DNA influence chromatin assembly in vitro.

“…For example, the 180-bp 1-globin spacing in chick erythroid cells (49), in which this gene is expressed, corresponds to the 180-bp spacing obtained in vitro. Similarly, the 196-bp spacing for the chicken ovalbumin gene in hen oviduct, in which this gene is expressed (3,46), corresponds to the in vitro value obtained (30). The chicken ovalbumin gene is not restricted to this value because in erythrocytes, it has a 210-bp nucleosome spacing periodicity, the bulk chromatin repeat (46,49) (Fig.…”

“…Moreover, activation in some cases appears to entail packaging into a large and stable nuclease-sensitive chromatin domain that exceeds by far the region of DNA undergoing transcription (6, 48). Additionally, active chromatin often contains irregular or altered nucleosome arrays (3,12,37,46,49) or exhibits a loss of nucleosome positioning with respect to DNA (7). It has not been adequately explained, however, how a localized activation event could prevent or alter the presumed default packaging of large stretches of DNA into inactive chromatin or, alternatively, how an active chromatin structure could propagate over a large distance.…”

Hormonal regulation of phosphoenolpyruvate carboxykinase gene expression is mediated through modulation of an already disrupted chromatin structure.

“…The rate of production of nucleosomal arrays by MNase digestion was constant throughout, indicating that the chromatin substrate did not, in all likelihood, significantly lack HI, the loss of which would have been expected to lead to a much greater rate of digestion. The finding that apparently normal nucleosomes were present, even on chromatin that was being transcribed vigorously, was interesting, since contradictory results have been reported in this regard (4,10,36,37,40). Clearly, gross nucleosomal disruption is not a sine qua non for transcription, although extremely vigorous transcription might be expected to lead to more-diffuse nucleosomal patterns.…”

“…On the other hand, it has also been argued that some particularly active genes may lose elements of normal nucleosomal arrangement (4,10,37). Genes that are transcriptionally active reside in a region of chromatin that is more sensitive to DNase I digestion than is bulk material (15,16,43).…”

The active immunoglobulin kappa chain gene is packaged by non-ubiquitin-conjugated nucleosomes.

“…In vitro the histones strongly inhibit transcription (1), yet the results of experiments employing immunoelectron microscopy (2), microinjection of antibodies (3), and nuclease digestion (4)(5)(6) suggest that nucleosomes are present on many transcriptionally active class II genes in vivo.…”