Structural details of the thermophilic filamentous bacteriophage PH75 determined by polarized Raman microspectroscopy.

The filamentous virus PH75, which infects the thermophile Thermus thermophilus, consists of a closed DNA strand of 6500 nucleotides encapsidated by 2700 copies of a 46-residue coat subunit (pVIII). The PH75 virion is similar in composition to filamentous viruses infecting mesophilic bacteria but is distinguished by in vivo assembly at 70 °C and thermostability to at least 90 °C. Structural details of the PH75 assembly are not known, although a fiber X-ray diffraction based model suggests that capsid subunits are highly α-helical and organized with the same symmetry (class II) as in the mesophilic filamentous phages Pfl and Pf3 [Pederson et al. (2001) J. Mol. Biol. 309, 401-421]. This is distinct from the symmetry (class I) of phages fd and M13. We have employed polarized Raman microspectroscopy to obtain further details of PH75 architecture. The spectra are interpreted in combination with known Raman tensors for modes of the pVIII main chain (amide I) and Trp and Tyr side chains to reveal the following structural features of PH75: (i) The average pVIII peptide group is oriented with greater displacement from the virion axis than peptide groups of fd, Pfl, or Pf3. The data correspond to an average helix tilt angle of 25° in PH75 vs 16° in fd, Pfl, and Pf3. (ii) The indolyl ring of Trp 37 in PH75 projects nearly equatorially from the subunit α-helix axis, in contrast to the more axial orientations for Trp 26 of fd and Trp 38 of Pf3. (iii) The phenolic rings of Tyr 15 and Tyr 39 project along the subunit helix axis, and one phenoxyl engages in hydrogen-bonding interaction that has no counterpart in either fd or Pfl tyrosines. Also, in contrast to fd, Pfl, and Pf3, the packaged DNA genome of PH75 exhibits no Raman anisotropy, suggesting that DNA bases are not oriented unidirectionally within the nucleocapsid assembly. The structural findings are discussed in relation to intrasubunit and intersubunit interactions that may confer hyperthermostability to the PH75 virion. A refined molecular model is proposed for the PH75 capsid subunit.