Nucleic Acids Research, 1980, Vol. 8, No. 5 1107-1119
© 1980
Articles |
On the base-stacking in the 5'-terminal cap structure of mRNA: a fluorescence study
*Faculty of Pharmaceutical Sciences, University of Tokyo Hongo, Bunkyo-ku, Tokyo 113 National Institute of Genetics Mishima 411 **Department of Life Chemistry, Tokyo Institute of Technology Nagatsuda, Yokohama 227, Japan
Received December 4, 1979.
The fluorescence at 370 nm of the 7-methylguanosine residue (m7G) is found to be quenched when the base residue is involved in a stacking interaction with the adenosine residue in the cap structure m7G5'pppA of an eukaryotic mRNA. On the basis of the observed degree of quenching, the amounts of the stacked and unstacked forms in the cap structure have been determined at various temperatures and pH's. It has been found that at pH 6.2 effective enthalpy and entropy in the unstacked 
stacked change are 
H° = –4.4 ± 0.1 kcal/mole and S° = – 14.3 ± 0.2 e.u., respectively. The pka value for the m7G residue is found to be 7.7 at 10°C and 7.3 at 30°C. The stacked structure seems to be less favourable in the deprotonated form that occurs in the higher pH solution. A similar analysis of some other cap structures indicates that the stacked form in m7G5' pppN structure is favourable if N is a purine nucleoside or a 2'-O-methylpyrimidine nucleoside but not for an unmethylated pyrimidine nucleoside.