Overproduction, purification, and characterization of DNA-binding protein P19 of bacteriophage PRD1

The early protein, P19, of bacteriophage PRD1 was purified after overexpression of the cloned gene, XIX, in Escherichia coli DH5 alpha cells. The purified protein binds as multimers to single-stranded DNA (ssDNA), and with a lower affinity to double-stranded DNA (dsDNA), without sequence-specificity. Two distinct P19-ssDNA complexes were discovered in gel- mobility-shift assays at different protein:DNA ratios. P19 was capable of fully protecting ssDNA against nuclease P1. Electron microscopy of protein P19-ssDNA complexes showed DNA molecules which were extensively coated with protein and whose contour length was clearly reduced by P19 binding. The results suggest that P19 binds to ssDNA with moderate cooperativity and are consistent with the DNA being wrapped around the P19 multimers.