Ultraviolet Absorbance and Circular Dichroism of Pf1 Virus: Nucleotide/Subunit Ratio of Unity, Hyperchromic Tyrosines and DNA Bases, and High Helicity in the Subunits

Abstract

Data have been obtained for the Pfl virion that establish its stoichiometry and conformational features of its DNA and its protein. The absorbance spectrum of the dissociated virus under alkaline denaturing conditions is fit exactly by spectra for DNA and protein at a mole ratio of one nucleotide per protein subunit. This result, together with three previous values by independent methods, establishes that the nucleotide/subunit ratio (n/s) of Pfl is unity. The absorbance spectrum of DNA in the intact native virus is assigned as the spectrum for heat denatured Pfl DNA, with ϵ(P) = 8400 M−1 cm−1 at 259 nm. The absorbance spectrum assigned to protein (two tyrosines) in the intact virus has 〈ϵ(Y)〉 = 2500 M−1 cm−1 per tyrosine at λmax of 281.5 nm

this is the most red-shifted and hyperchromic tyrosine spectrum known. The CD spectrum of the intact virus from 250 to 320 nm has no apparent DNA contribution, but has a strong contribution from the red-shifted tyrosine(s). The CD spectrum from 185 to 250 nm has the shape of α-helical CD reference spectra, but is perceptibly blue-shifted, with a crossover from negative to positive ellipticity at 199.7 nm, and it has very high amplitudes (e.g. [θ207.5nm] = -44 000 deg cm2 dmol−1). This spectrum indicates completely helical protein in the virus, with a predominance of α-helix and perhaps some 310-helix. The unit n/s ratio, the high absorbance and negligible near-UV CD for the DNA bases, and the high amplitudes for the helical protein are critical input data for the determination of Pf1 virus structure. © 1994, American Chemical Society. All rights reserved.