title: Unwinding of unnatural substrates by a DNA helicase.
abstract: Helicases separate double-stranded DNA into single-stranded DNA intermediates that are required during replication and recombination. These enzymes are believed to transduce free energy available from ATPase activity to unwind the duplex and translocate along the nucleic acid lattice. The nature of enzyme-substrate interactions between helicases and duplex DNA substrates has not been well-defined. Most helicases require a single-stranded DNA overhang adjacent to duplex DNA in order to initiate unwinding. The strand containing the overhang is referred to as the loading strand whereas the complementary strand is referred to as the displaced strand. We have investigated the interactions between a DNA helicase and the DNA substrate by replacing the displaced strand with a nucleic acid mimic, peptide nucleic acid (PNA). PNA is capable of forming duplex structures with DNA according to Watson-Crick base pairing rules, but contains a N-(2-aminoethyl)glycine backbone in place of the deoxyribose phosphates. The PNA-DNA hybrids had higher melting temperatures than their DNA-DNA counterparts. Dda helicase, from bacteriophage T4, was able to unwind the DNA-PNA substrates at similar rates as DNA-DNA substrates. The results indicate that the rate-limiting step for unwinding is relatively insensitive to the chemical nature of the displaced strand and the thermal stability of oligonucleotide substrates.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205, USA.
authors.forename: A J
authors.initials: AJ
authors.lastname: Tackett
authors.forename: P D
authors.initials: PD
authors.lastname: Morris
authors.forename: R
authors.initials: R
authors.lastname: Dennis
authors.forename: T E
authors.initials: TE
authors.lastname: Goodwin
authors.forename: K D
authors.initials: KD
authors.lastname: Raney
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D017122:Bacteriophage T4 / Q000201:enzymology; D004265:DNA Helicases / Q000737:chemistry* / Q000378:metabolism; D004277:DNA, Single-Stranded / Q000737:chemistry; D004279:DNA, Viral / Q000737:chemistry; D004789:Enzyme Activation; D006358:Hot Temperature; D009692:Nucleic Acid Heteroduplexes / Q000737:chemistry; D009841:Oligonucleotides / Q000737:chemistry; D020135:Peptide Nucleic Acids / Q000737:chemistry; D012367:RNA, Viral / Q000737:chemistry; D013379:Substrate Specificity; D014764:Viral Proteins
chemical_list: D004277:DNA, Single-Stranded; D004279:DNA, Viral; D009692:Nucleic Acid Heteroduplexes; D009841:Oligonucleotides; D020135:Peptide Nucleic Acids; D012367:RNA, Viral; D014764:Viral Proteins; D000251:Adenosine Triphosphatases; D004265:DNA Helicases; C077401:dda DNA helicase protein, Bacteriophage T4