Eclipse kinetics as a probe of quaternary structure in bacteriophage phi X174

The extracellular form of bacteriophage phi X174 consists of single-stranded DNA within an icosahedral capsid, which has short spikes at each of its vertices. Each spike is composed of gene G and H proteins, while the capsid itself consists of gene F protein. Since several molecules of gene H protein are injected into the cell along with the DNA, specific protein--protein and DNA--protein interactions must be broken when the genome exits and leaves an intact capsid structure at the receptor site. To demonstrate this we examined the eclipse (DNA ejection) reaction with two types of phi X174 mutants. The first contains missense mutations in a capsid or spike protein gene, and the second involves insertions or deletions in non-coding regions of the DNA. Using an improved procedure, the eclipse rate in vivo of the eclipse mutants Fcs70 has been redetermined over a larger temperature range than in previous studies. The three- to fivefold decrease in rate between 37 degrees C and 25 degrees C is due to an increase in both the enthalpy and entropy of activation when compared to the wild-type values of these kinetic parameters. This missence mutation also confers an increase in virus stability in 2 to 3 M-urea. In contrast to this, inserting 163 bases into the length of DNA packaged within the phi X174 capsid does not lead to a detectable change in eclipse rate over the same temperature range. yet this insertion into the J--F intercistronic region imparts a significant decrease in virus stability in urea. These results suggest that a specific set of non-covalent interactions is involved in phi X174 DNA ejection. This is supported by the small (50%), but significant, increase in eclipse rate that occurs when 27 bases are deleted from the J--F intercistronic region. The latter effect must be base-sequence-specific since no change in rate is observed when only seven of the 27 bases are deleted. Thus, the kinetics of the phi X174 eclipse reaction can be used as a sensitive probe of quaternary structure by correlating the change in reaction rate with alterations in amino acid and base sequences in the structural components of the virus.