Control of DNA Structure by Proteins

Abstract

DNA-binding proteins (Alberts and Frey 1970) play an important role in the regulation of nucleic acid metabolism in cells and in the life cycles of bacteriophages and animal viruses. They can influence the structure of nucleic acids and can affect enzymatic activities either directly by associating with other proteins or indirectly by binding to DNA. Proteins showing cooperative binding to single-stranded (SS) DNA denature the double helix in low salt by binding to frayed ends of the DNA. In addition, the double-stranded state of DNA is under the control of enzymes. In Escherichia coli, these include a nicking-closing enzyme (Wang 1971), DNA gyrase (Gellert et al. 1976), and ATP-dependent enzymes capable of separating the two strands of the DNA double helix. Three DNA-dependent ATPases that unwind duplex DNA, referred to here as DNA helicases (previously known as DNA-unwinding enzymes [Abdel-Monem et al. 1976]), will be discussed in this article. Two of these enzymes were purified from E. coli and the third was purified from cells infected with T4 bacteriophage. Still another DNA-dependent ATPase that unwinds DNA is the product of the rep gene of E. coli. Mutations in rep are not lethal for the cell but block replication of double-stranded replicative form (RF) DNA of small bacteriophages such as ϕX174 (Denhardt et al. 1972).

DNA-BINDING PROTEINS
The role of phage DNA-binding proteins in the cell was deduced from genetic defects. The absence of phage fd DNA-binding protein, which is encoded by gene V, leads to an accumulation of RF DNA...