Bacteriophage φ6-Structure Investigated by Fluorescence Stokes Shift Spectroscopy

The Stokes shift of tryptophan (Trp) fluorescence from layers of the lipid‐containing bacteriophage φ6 is compared to determine the relative effect of the layers on virus hydrophobicity. In the inner most layer, the empty procapsid (PC) which contains 80–90% of the virion Trp residues, λmax = 339.8 nm. The PC emission is substantially more redshifted than the other φ6 layers and nearer to that of the Pseudomonad host cell than the other φ6 layers. The Trp emission from the nucleocapsid (NC) with λmax = 337.4 nm, is blueshifted by 2.4 nm relative to the PC although the number of Trp in the NC is identical to the PC. This shift represents an increase in Trp hydrophobicity, likely a requirement for the maintenance of A‐form doubled‐stranded RNA. Fluorescence from the completely assembled virion indicates it is in a considerably more hydrophobic environment with λmax = 330.9 nm. Density measurements show that the water content in the NC does not change during envelope assembly, therefore the blueshifted φ6 emission suggests that the envelope changes the PC environment, probably via the P8 layer. This change in hydrophobicity likely arises from charge redistribution or envelope‐induced structural changes in the PC proteins. Tryptophan emission from the different layers of the bacteriophage ϕ6. Differences in Stokes shift correspond to differences in hydrophobicity of the layers. The hydrophobicity of the procapsid (PC) is similar to that of the pseudomonad host cell. Genome packaging of the PC increases accommodate A‐form dsRNA and hydrophobicity. Assembly of the envelope greatly increases the hydrophobicity of the PC proteins, indicating a strong interaction between the envelope and the inner proteins.