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DnaB helicase is an enzyme in bacteria which opens the replication fork during DNA replication.Although the mechanism by which DnaB both couples ATP hydrolysis to translocation along DNA and denatures the duplex is unknown, a change in the quaternary structure of the protein involving dimerisation of the N-terminal domain has been observed and may occur during the enzymatic cycle. [1]
Helicases move incrementally along one nucleic acid strand of the duplex with a directionality and processivity specific to each particular enzyme. Helicases adopt different structures and oligomerization states. Whereas DnaB-like helicases unwind DNA as ring-shaped hexamers, other enzymes have been shown to be active as monomers or dimers.
DnaC helps the helicase to bind to and to properly accommodate the ssDNA at the 13 bp region; this is accomplished by ATP hydrolysis, after which DnaC is released. Single-strand binding proteins (SSBs) stabilize the single DNA strands in order to maintain the replication bubble. DnaB is a 5'→3' helicase, so it travels on the lagging strand.
In terms of structure, the replisome is composed of two replicative polymerase complexes, one of which synthesizes the leading strand, while the other synthesizes the lagging strand. The replisome is composed of a number of proteins including helicase, RFC, PCNA, gyrase/topoisomerase, SSB/RPA, primase, DNA polymerase III, RNAse H, and DNA ligase.
The DnaC helicase loader then interacts with the DnaA bound to the single-stranded DNA to recruit the DnaB helicase, [9] which will continue to unwind the DNA as the DnaG primase lays down an RNA primer and DNA Polymerase III holoenzyme begins elongation. [10]
In prokaryotes, DnaA hydrolyzes ATP in order to unwind DNA at the oriC. This denatured region is accessible to the DnaB helicase and DnaC helicase loader. Single-strand binding proteins stabilize the newly formed replication bubble and interact with the DnaG primase. DnaG recruits the replicative DNA polymerase III, and replication begins.
This suggests that the DnaB helicase may remain stably associated at RFs and serve as a nucleation point for the competent holoenzyme. In vitro single-molecule studies have shown that Pol III* has a high rate of RF turnover when in excess, but remains stably associated with replication forks when concentration is limiting. [30] Another single ...
DNA helicases are responsible for unwinding the double-stranded DNA during chromosome replication. Helicases in eukaryotic cells are remarkably complex. [106] The catalytic core of the helicase is composed of six minichromosome maintenance (Mcm2-7) proteins, forming a hexameric ring. Away from DNA, the Mcm2-7 proteins form a single ...