Emeritus Faculty, Acad Council, Biochemistry
We study Herpes simplex virus type 1 as a model eukaryotic chromosome for the analysis of eukaryotic DNA replication and recombination.
The UL9 protein of herpes simplex virus type 1 (HSV-1) binds specifically to the HSV-1 oriS and oriL origins of replication, and is a DNA helicase and DNA-dependent NTPase. In this study electron microscopy was used to investigate the binding of UL9 protein to DNA fragments containing oriS. In the absence of ATP, UL9 protein was observed to bind specifically to oriS as a dimer or pair of dimers, which bent the DNA by 35 degrees +/- 15 degrees and 86 degrees +/- 38 degrees respectively, and the DNA was deduced to make a straight line path through the protein complex. In the presence of 4 mM ATP, binding at oriS was enhanced 2-fold, DNA loops or stem-loops were extruded from the UL9 protein complex at oriS, and the DNA in them frequently appeared highly condensed into a tight rod. The stem-loops contained from a few hundred to over one thousand base pairs of DNA and in most, oriS was located at their apex, although in some, oriS was at a border. The DNA in the stem-loops could be stabilized by photocrosslinking, and when Escherichia coli SSB protein was added to the incubations, it bound the stem-loops strongly. Thus the DNA strands in the stem-loops exist in a partially paired, partially single-stranded state presumably making them available for ICP8 binding in vivo. These observations provide direct evidence for an origin specific unwinding by the HSV-1 UL9 protein and for the formation of a relatively stable four-stranded DNA in this process.
View details for Web of Science ID A1996UF65800030
View details for PubMedID 8612599
View details for PubMedCentralID PMC450087
Whole-cell extracts of herpes simplex virus type 1-infected human cells (293 cells) can promote the rolling circle replication of circular duplex DNA molecules. The products of the reaction are longer than monomer unit length and are the result of semiconservative DNA replication by the following criteria: (i) resistance to DpnI and susceptibility to MboI restriction enzymes, (ii) shift in density on a CsCl gradient of the products synthesized in the presence of bromo-dUTP to a position on the gradient consistent with those of molecules composed mainly of one parental DNA strand and one newly synthesized DNA strand, and (iii) the appearance in the electron microscope of molecules consisting of duplex circles with multiunit linear appendages, a characteristic of a rolling circle mode of DNA replication. The reaction requires ATP and is dependent on herpes simplex virus type 1-encoded DNA polymerase.
View details for Web of Science ID A1996TP52600055
View details for PubMedID 8551573
We had previously demonstrated that the herpes simplex virus 1 (HSV-1) single-stranded DNA-binding protein (ICP8) can specifically stimulate the helicase activity of the HSV-1 origin-binding protein (UL9). We show here that this functional stimulation is a manifestation of a tight interaction between UL9 protein and ICP8. By using protein-affinity chromatography, we have demonstrated the specific binding of purified UL9 protein to immobilized ICP8 and vice versa. Furthermore, ICP8 is specifically retained by a column on which the C-terminal 37-kDa DNA-binding domain of the UL9 protein was immobilized. The interaction between ICP8 and the DNA-binding domain of the UL9 protein was confirmed by cochromatography of the two proteins. These results suggest that the UL9 protein and ICP8 form a tight complex that functions in origin recognition and unwinding.
View details for Web of Science ID A1993LX75000031
View details for PubMedID 8397405
The herpes simplex virus type 1 (HSV-1) origin binding protein, the product of the UL9 gene, catalyzes the unwinding of DNA in the 3'-5' direction. Helicase activity is coupled to the hydrolysis of ATP or dATP and to a lesser extent to CTP, dCTP, or UTP. It requires a divalent cation (Mg2+ > Mn2+ > Ca2+) with an optimum at 2.5 mM MgCl2. Activity is optimal at high pH (8.5-9.5) and high temperature (45 degrees C) and is inhibited at ionic strengths > 50 mM NaCl. The helicase activity is specifically stimulated by the HSV-1-encoded single-stranded DNA-binding protein, ICP8, which increases both the rate and extent of helicase activity. Helicase action appears to be stoichiometric, requiring a DNA-dependent assembly of a multimeric UL9 protein complex. Under optimal conditions, the rate of DNA unwinding is approximately 75 base pairs/min.
View details for Web of Science ID A1993KG07700069
View details for PubMedID 8380408