ORIENTATIONS OF TYROSINE-21 AND TYROSINE-24 IN COAT SUBUNITS OF FF FILAMENTOUS VIRUS - DETERMINATION BY RAMAN LINEAR INTENSITY DIFFERENCE SPECTROSCOPY AND IMPLICATIONS FOR SUBUNIT PACKING

Virions of the Ff group of bacteriophages (fd, fl, M13) are morphologi cally identical filaments (similar to 6-nm diameter x similar to 880-n m length) in which a covalently closed, single-stranded DNA genome is sheathed by similar to 2700 copies of a 50-residue alpha-helical subun it (pVIII). Orientations of pVIII tyrosines (Tyr(21) and Tyr(24)) with respect to the filament axis have been determined by Raman linear int ensity difference (RLID) spectroscopy of flow-oriented mutant virions in which the tyrosines were independently mutated to methionine. The r esults show that the twofold axis of the phenolic ring (C1-C4 line) of Tyr(21) is inclined at 39.5 +/- 1.4 degrees from the virion axis, and that of Tyr(24) is inclined at 43.7 +/- 0.6 degrees. The orientation determined for the Tyr(21) phenol ring is close to that of a structura l model previously proposed on the basis of fiber x-ray diffraction re sults (Protein Data Bank, identification code 1IFJ). On the other hand , the orientation determined for the Tyr(24) phenol ring differs from the diffraction-based model by a 40 degrees rotation about the C alpha -C beta bond. The RLID results also indicate that each tyrosine mutati on does not greatly affect the orientation of either the remaining tyr osine or single tryptophan (Trp(26)) of pVIII. On the basis of these r esults, a refined model is proposed for the coat protein structure in Ff.