The Molecular Weight of a Fast Sedimenting MS2 RNA Form H. SLEGERS and W. FIERS [Article in Biopolymers, 9, 1373 (1970)l
I n a previous publication we described conditions for the formation of a fast sedimenting structure formed by the reaction of formaldehyde with MS2 RNA a t acidic pH.1 The molecular weight of this 37.5 S structure was estimated on the basis of the Scheraga-
Mandellcern relation:
By assuming a partial specific volume of 0.55 cc/g for the reacted RNA2 we calculated molecular weights of 1.66 X 106 to 2.14 X 106 corresponding to 0-values of 2.5 X 106 and 2.12 X 106, respectively. On this basis, we believed that the 37.5 S form was in fact a dimer. We have now directly measured the partial specific volume by means of the "Digitale Dichtemessung DMA 02," an apparatus described in detail by Kratky and collaborator^.^ The ;-value for CHzO-reacted RNA amounts to only 0.44 C C / ~. ~ The recalculated molecular weights, using the above mentioned p-values, and the experimental values for SZO,~, [ q ] and V , are 1.09 X lo6 and 1.37 X 106. The mean value is in good agreement with the molecular weights of 1.18 X 106 and I . 1 3 X 106 obtained by light-scattering with blue and green light, re~pect.ively.~ We c mclude that the fast sedimenting striict.ure, previously reported as a dimer, is in fact a contracted monomer.