Chromosomal Organization: Nucleoids, Chromosomal Folding, and DNA Topology
The bacterial chromosome (nucleoid), unlike the eukaryotic nucleus, is not bounded by a membrane. Nevertheless, it is a distinct structure that exhibits lobules when multiple replication forks occur and an elongated shape that may arise from coupled transcription-translation taking place at the nucleoid periphery. Since the nucleoid can be isolated with its DNA intact, a variety of topological experiments have been possible. Most have focused on DNA supercoiling, which is controlled largely by the enzymatic activity of DNA gyrase. A second enzyme, DNA topoisomerase I, acts as a safety valve to prevent the accumulation of excess supercoiling. Since negative supercoiling levels respond to changes in the extracellular environment, and since many activities of DNA are affected by supercoiling, supercoiling is potentially important as a component of global regulatory circuits. The nucleoid contains a variety of small proteins, some of which appear to increase the flexibility of DNA. The protein HU facilitates both gyrase activity and site-specific protein-DNA interactions. As a consequence of replication, interlinking occurs between daughter chromosomes, and a decatenase, topoisomerase IV, appears to be responsible for separating them so chromosome segregation can occur.
KeywordsReplication Fork Safety Valve Bacterial Chromosome Oxolinic Acid Daughter Chromosome
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