The crystal structure of the dimeric gene V protein of bacteriophage fl was determined using multiwavelength anomalous diffraction on the selenomethioninecontaining wild-type and isoleucine-47 methionine mutant proteins with x-ray diffraction data phased to 2.5 A resolution.The structure of the wild-type protein has been refined to an R factor of 19.2% using native data to 1.8 A resolution. The structure of the gene V protein was used to obtain a model for the protein portion of the gene V protein-single-stranded DNA complex.Gene V protein of bacteriophage fltt is a member of a class of proteins involved in DNA replication that bind to singlestranded nucleic acids with high affinity and cooperativity but little sequence specificity (1-4). Gene V protein coats the single-stranded DNA (ssDNA) intermediate in bacteriophage fl DNA replication, forming an ordered superhelical protein-DNA complex (1,(5)(6)(7)(8). This protein-DNA complex facilitates packaging of the ssDNA into new phage particles. Gene V protein also binds with some specificity to a translational operator sequence on phage fl gene II mRNA (9-12).The gene V protein provides a general model for proteinssDNA interactions that are strong, yet not sequencespecific. Gene V protein binding to single-stranded nucleic acids and to oligonucleotides has been studied using chemical modification, spectroscopic techniques, and mutagenesis (13-24). The structure of the protein-ssDNA complex has been studied using electron microscopy and solution scattering methods and is found to consist of a regular left-handed superhelix in which the gene V protein dimers are arrayed on the outside of the superhelix and the ssDNA strands are inside (8,25). The structure of the gene V protein is also of interest because its small size and the large number of mutants available have made it a useful model for determining effects ofamino acid substitutions on protein stability and function (23,26,27).A model for the crystal structure of the wild-type (WT) gene V protein has been reported (28), but recent NMR studies have demonstrated that the positions of amino acids involved in the segments of antiparallel (-structure are not compatible with those in the model (21), and a new determination of the structure was necessary. The multiwavelength anomalous diffraction (MAD) technique (29) was ideally suited for this purpose, as the WT gene V protein contains two methionine residues (Met-1 and Met-77) that might be substituted in vivo in Escherichia coli by selenomethionine. Here we report the determination of the gene V protein structure using the MAD technique, the refinement of the structure using x-ray diffraction data on the WT gene V proteinAt and a model for protein-protein contacts in the gene V protein-ssDNA superhelical complex.
MATERIALS AND METHODSModeling of the Protein Portion of Gene V Protein-ssDNA Complex. The gene V protein dimer was placed with its internal two-fold axis of symmetry perpendicular to the axis of the superhelix to be generated and Phe-73 pointing ei...