Unwinding protein

Figure 36-17. The generation of "replication bubbles" during the process of DNA synthesis. The bidirectional replication and the proposed positions of unwinding proteins at the replication forks are depicted.

Duguet, M. and de Recondo, A.M. (1978) A deoxyribonucleic acid unwinding protein isolated from regenerating rat liver. Physical and functional properties. J. Biol. Chem. 253, 1660-1666. 

Unwinding proteins. Proteins that help to unwind double-stranded DNA during DNA replication. 

They are unwinding proteins. They attach themselves to one strand of the DNA and promote the separation of the two strands of DNA. 

FIGURE 1.15 Electron density from a monoclinic unit cell of the Gene 5 DNA Unwinding Protein crystal, lying between y = 0.125 and y = 0.250, is projected onto a single plane. Superimposed upon this electron density is a portion of the atomic model of the Gene 5 Protein. Electron density planes, like those shown here, are the images obtained directly by X-ray diffraction from computed Fourier... 

We have mentioned how DNA polymerase I from Escherichia coli extends the DNA chain, but how does it initiate the chain A clue to the mechanism was obtained when it was observed that rifampicin, an inhibitor of RNA polymerase, also inhibits the conversion of a phage single stranded into a double stranded DNA circle. This suggested that DNA synthesis is initiated by RNA priming. It is now established that the initiation of DNA synthesis by DNA polymerase I requires RNA and DNA polymerase, ribose and deoxyribose triphosphates, an unwinding protein and other factors whose roles are still unclear. The initiation step seems to involve a transcriptional operation by RNA polymerase. The newly synthesized RNA serves as primer for DNA polymerase I, yielding an RNA-DNA complex and the primary RNA is excised by a nuclease [236, 237]. 

It has a molecular weight of 60,000 to 90,000 daltons and requires NH4, Mg2, and native DNA for activity. It interacts with the DNA unwinding protein and this reaction accelerates the rate of DNA synthesis. The synthesis proceeds from the 5 toward the 3 direction. RNA polymerase generates the primer. 

A number of enzymes which bind polyanions such as DNA may be separated broadly into those which recognize special sequences with higher affinity than "bulk DNA" and those which bind nonspecifically. The first group includes RNA polymerases, repressors, cAMP receptor protein (CRP), the rho factor, restriction endonucleases, and methy-lases the second group includes histones and DNA-unwinding proteins. 

Herrick, G., and Alberts, B., 1976a, Purification and physical characterization of nucleic acid helix-unwinding proteins from calf thymus, /. Biol. Chem. 251 2124. 

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