Purification and enzymological characterization of DNA-dependent ATPase IV from the Novikoff hepatoma

DNA-dependent adenosinetriphosphatase A: immunoaffinity purification and characterization of immunological reagents

We describe an immunoaffinity purification of DNA-dependent ATPase A from fetal calf thymus. The rapid purification increases the yield of enzymatically active enzyme approximately 4-fold, with up to a 7-fold increase in specific activity, and significantly improves the yield of a higher molecular weight species of ATPase A. In the presence of a denatured calf thymus DNA effector, the immunoaffinity-purified enzyme has a specific activity that is more than 10-fold higher than reported for any other eukaryotic DNA-dependent ATPase and 100-fold higher than most others. The improvement in yield has allowed several polypeptides to be identified using monoclonal antibodies, and these polypeptides are demonstrated to be structurally related by partial peptide mapping with N-chlorosuccinimide. The preferred DNA effector for ATP hydrolysis continues to be a DNA primer-template junction with an adjacent stretch of single-stranded DNA. We have used the immunoaffinity-purified enzyme to develop additional stable murine hybridoma monoclones, resulting in a bank of antibodies that recognize a number of different epitopes. All of the monoclonal antibodies react with both calf thymus DNA-dependent ATPase A and bacteriophage T4 gene 44 protein, a DNA-dependent ATPase essential for DNA replication in the bacteriophage T4 system. These monoclonal antibodies should facilitate the development of our understanding with respect to the role and regulation of DNA-dependent ATPases in eukaryotic DNA replication.

The single-stranded DNA-binding protein of Escherichia coli

The single-stranded DNA-binding protein (SSB) of Escherichia coli is involved in all aspects of DNA metabolism: replication, repair, and recombination. In solution, the protein exists as a homotetramer of 18,843-kilodalton subunits. As it binds tightly and cooperatively to single-stranded DNA, it has become a prototypic model protein for studying protein-nucleic acid interactions. The sequences of the gene and protein are known, and the functional domains of subunit interaction, DNA binding, and protein-protein interactions have been probed by structure-function analyses of various mutations. The ssb gene has three promoters, one of which is inducible because it lies only two nucleotides from the LexA-binding site of the adjacent uvrA gene. Induction of the SOS response, however, does not lead to significant increases in SSB levels. The binding protein has several functions in DNA replication, including enhancement of helix destabilization by DNA helicases, prevention of reannealing of the single strands and protection from nuclease digestion, organization and stabilization of replication origins, primosome assembly, priming specificity, enhancement of replication fidelity, enhancement of polymerase processivity, and promotion of polymerase binding to the template. E. coli SSB is required for methyl-directed mismatch repair, induction of the SOS response, and recombinational repair. During recombination, SSB interacts with the RecBCD enzyme to find Chi sites, promotes binding of RecA protein, and promotes strand uptake.