Electron Microscopy of Protein-Nucleic Acid Complexes: Uniform Spreading and Determination of Helix Handedness

There are a number of proteins involved in DNA replication, recombination, or repair that bind stoichiometrically to single DNA strands of any nucleotide sequence, and which in some cases can also bind to single-stranded RNA. The best known examples are the ssb protein of E. coli , the gene 32 protein of phage T4, and the gene 5 protein of the M 13/fd/f 1 filamentous viruses (1 –3 ). Complexes formed by these proteins contain protein bound to the nucleic acid at defined ratios of the number of nucleotides per molecule of bound protein; the ratios are determined by the interactive properties of the protein. These ratios, and the structures of the complexes that are formed, may vary with factors such as changes in solution conditions that alter the binding properties of the proteins.