Bacteriophage lambda gpNu1 and Escherichia coli IHF proteins cooperatively bind and bend viral DNA: implications for the assembly of a genome-packaging motor

Terminase enzymes are common to both prokaryotic and eukaryotic double-stranded DNA viruses and are responsible for packaging viral DNA into the confines of an empty procapsid shell. In all known cases, the holoenzymes are heteroligomers composed of a large subunit that possesses the catalytic activities required for genome packaging and a small subunit that is responsible for specific recognition of viral DNA. In bacteriophage lambda, the DNA recognition protein is gpNu1. The gpNu1 subunit interacts with multiple recognition elements within cos, the packaging initiation site in viral DNA, to site-specifically assemble the packaging machinery. Motor assembly is modulated by the Escherichia coli integration host factor protein (IHF), which binds to a consensus sequence also located within cos. On the basis of a variety of biochemical data and the recently solved NMR structure of the DNA binding domain of gpNu1, we proposed a novel DNA binding mode that predicts significant bending of duplex DNA by gpNu1 (de Beer et al. (2002) Mol. Cell 9, 981-991). We further proposed that gpNu1 and IHF cooperatively bind and bend viral DNA to regulate the assembly of the packaging motor. Here, we characterize cooperative gpNu1 and IHF binding to the cos site in lambda DNA using a quantitative electrophoretic mobility shift (EMS) assay. These studies provide direct experimental support for the long presumed cooperative assembly of gpNu1 and IHF at the cos sequence of lambda DNA. Further, circular permutation experiments demonstrate that the viral and host proteins each introduce a strong bend in cos-containing DNA, but not nonspecific DNA substrates. Thus, specific recognition of viral DNA by the packaging apparatus is mediated by both DNA sequence information and by structural alteration of the duplex. The relevance of these results with respect to the assembly of a viral DNA-packaging motor is discussed.