Structural transitions in bacteriophages Pf3 and Xf

Laser-Raman spectra of filamentous bacteriophages Pf3 and Xf show that the coat protein subunits in each phage are predominantly alpha-helical. However, the subunits of both viruses are converted to beta-sheet structures by raising the temperature. The transition temperature depends upon phage concentration and solution ionic strength. Cooling of the solutions returns both viruses from their beta-states to alpha-states similar to the native structures. Through prolonged heating at temperatures above 80 degrees C, the "beta-sheet" of Pf3 can be irreversibly converted to a second alpha-helical structure which is distinct from the structure in the native phage. The spectra also show that aromatic amino acid residues of Pf3 and Xf undergo dramatic changes in molecular environment in the alpha in equilibrium with beta transitions. The Raman spectra provide little of either Pf3 or Xf, although bands characteristic of classical A-type and B-type structures are not observed. The combined results are of significance with regard to both the interpretation of fiber X-ray diffraction patterns of filamentous phages and the mechanism of virion assembly.