Structural characterization of the filamentous bacteriophage PH75 from Thermus thermophilus by raman and UV-resonance raman spectroscopy

The filamentous bacteriophage PH75, which infects the thermophile T. thermophilus, assembles in vivo at 70 °C and is stable to at least 90 °C. Although a high-resolution structure of PH75 is not available, the virion is known to comprise a closed single-stranded (ss) DNA circle of 6500 nucleotides sheathed by a capsid comprising 2700 copies of a 46-residue subunit (pVIII). Here, we employ Raman and UV-resonance Raman (UVRR) spectroscopy to identify structural details of the pVIII and DNA constituents of PH75 that may be related to the high thermostability of the native virion assembly. Analysis of the Raman amide I and amide III signatures reveals that the capsid subunit secondary structure is predominantly (87%) α-helical but contains a significant number of residues (6 ± 1 or 13 ± 3%) differing from the canonical α-helix. This minor structural component is not apparent in capsid subunits of the mesophilic filamentous phages, fd, Pf1, and Pf3, previously examined at similar spectral resolution. The Raman...