DNA Helicases
DNA replication requires opening of the double-strand DNA by an enzyme called DNA helicase. The most likely candidate for DNA strand opening in humans at the replication fork is the Mcm2-7 complex consisting of a hexamer of six individual protein subunits. Presumably, several other DNA helicases pursue additional roles during DNA replication. Among these are the so called RecQ helicases that have also been suggested to take part in replication comparable to the situation in bacteria, where the RecQ helicase together with another protein named RecJ resurrects stalled or collapsed replication forks.
In contrast to bacteria higher eukaryotes possess five different RecQ helicases, three of which have been attributed to human diseases. A defect in ReqQL2 or Werner (WRN) helicase leads to premature aging and a largely increased susceptibility to cancer. Defects in RecQL3 or Bloom (Blm) helicase cause craniofacial malformations, small stature, telangiectasia, immunodeficiencies, malignancies, and again premature aging. Defects in RecQL4 helicase cause the Rothmund Thomson syndrome (RTS) that comes along with abnormal skin pigmentation, telangiectasia, hypogonadism, ear problems, and osteosarcomas, whereas for RecQL1 and RecQL5 deficiencies there are no reported diseases until now.
The Grosse group has recently shown that the WRN helicase preferably unwinds mixed RNA-DNA strands, such as they occur on the lagging strand of DNA replication. Interestingly, this reaction is stimulated by another enzyme, DNA helicase II (NDH II), whose molecular action hitherto was mainly assigned to RNA metabolism. Moreover, NDH II stimulates the 3´-5´ exonuclease of WRN and strongly inhibits WRN's branch migration activity, which indicates that these two enzymes may also cooperate in vivo. Based on these findings NDH II may act as an anti-recombinase by diminishing WRN's actions in D-loop unwinding and branch migration. On the other hand NDH II may stimulate WRN-induced DNA rearrangements at ongoing or stalled replication forks, where WRN may be involved in the unwinding of Okazaki fragments.
Interestingly, NDH II also seems to bind to the replication initiation and elongation factor Cdc45p (see Replication Initiation). Therefore we speculate that Cdc45 may attract other helicases to stalled replication forks when it lost its usual contact to the ongoing Mcm2-7 complex, e.g. because one of the replicative polymerases halts at a damaged site. We are currently testing this hypothesis by biochemical and cell biological means.
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