RESUMO
DNA polymerase-alpha-primase may be isolated from pea shoot tip cells as a large (1.25 x 10(6) Da) multi-protein complex. The complex exhibits several enzyme activities and also binds to DNA. One of the DNA-binding activities has been purified as a 42 kDa polypeptide. The binding of this polypeptide to linear DNA fragments and to open circular plasmids has been studied by electron microscopy. The protein binds to restriction enzyme-generated cohesive ends of linear fragments and also exhibits some interstitial binding. Binding at the ends of linear molecules is very markedly reduced if the molecules are previously treated with S1 nuclease. The protein also binds to open circular plasmids; the number of binding sites is increased by exposing the plasmids to gamma-irradiation prior to the DNA-protein interaction. In these experiments, the number of protein units bound is directly related to the radiation dose. With both linear and open circular molecules, binding of the protein to the DNA leads to an apparent shortening of the DNA molecule. These observations, taken with the finding that the protein does not bind to completely single-stranded DNA, lead to the suggestion that the protein binds to double-stranded-single-stranded (ds-ss) junctions in DNA and that binding causes the DNA to wrap round the protein.
