ATPDependent Chromatin Remodeling Complexes

The presence of enzymes that can alter the structure of chromatin was suggested by yeast genetic studies that identified a number of genes, called SWI and SNF genes. These genes are required for multiple transcriptional activation events. A key breakthrough came when it was discovered that yeast cells could compensate for a deficiency in these SWI and SNF gene products by altering their chromatin structure. This led to the hypothesis that SWI and SNF genes are involved in the regulation of chromatin structure. It is now known that SWI and SNF proteins form a large, multisubunit complex, termed SWI/SNF, that can hydrolyze ATP and use the energy thus generated to alter chromatin structure.

Similar proteins that can hydrolyze ATP are present throughout the eukaryotic kingdom, and these form related multiprotein enzymes that also possess chromatin-remodeling properties. The mechanism by which these complexes alter the chromatin structure is unclear, but it is likely that the enzymes break or loosen the linkage between histone and DNA in a manner that increases the mobility and flexibility of the DNA wrapped around the histone core. It is important to keep in mind that these enzymes can be involved both in gene-activation events, by facilitating the binding of transcriptional activators, and in gene-repression events, perhaps by facilitating the binding of a transcriptional repressor or by directly promoting compaction of the chromatin structure. see also Cell Cycle; Cell, Eukary-otic; Chromosomal Banding; Chromosome, Prokaryotic; DNA; Evolution of Genes; Gene; Gene Expression: Overview of Control; In situ Hybridization; Meiosis; Mitosis; Mosaicism; Repetitive DNA Elements; Replication; Telomere; X Chromosome; Y Chromosome.

Cynthia Guidi and Anthony N. Imbalzano

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Chromosome, Prokaryotic

The bacterial or prokaryotic chromosome differs in many ways from that of the eukaryote. The term "eukaryote" comes from the Greek and means "true nucleus." Eukaryotic cells have a double membrane (the nuclear membrane) surrounding the nucleus, the organelle that contains several chromosomes. In contrast, the term "prokaryote" means "primitive nucleus," and, indeed, cells in prokaryotes have no nucleus. Instead, the prokaryotic chromosome is dispersed within the cell and is not enclosed by a separate membrane.

This dispersed chromosome is called the bacterial "nucleoid," which can be seen in electron micrographs of thin sections, as shown in Figure 2. Although bacteria (now called eubacteria) are highly diverse, the prototypical bacterial species is Escherichia coli, which has served as a model organism for genetic, biochemical, and biotechnological research for many decades.

The E. coli chromosome is a single circle. Because the single DNA molecule forming the chromosome is so long (about 4.6 million base pairs), it is easily broken when researchers try to isolate it. However, in the early 1960s, the Australian biochemist John Cairns was able to gently lyse E. coli cells without breaking the chromosome. He was interested in chromosomal replication and had labeled the DNA with tritium (3H), a radioactive form of hydrogen. Autoradiograms of the DNA demonstrated that the bacterial chromosome is a circular molecule. While the vast majority of bacterial species possess a single unique chromosome, there are a few rare species, nuclear membrane membrane surrounding the nucleus lyse break apart

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