Thermodynamic parameters for an expanded nearest-neighbor model for the formation of RNA duplexes with single nucleotide bulges

Thirty-four RNA duplexes containing single nucleotide bulges were optically. melted, and the thermodynamic parameters DeltaHdegrees, DeltaSdegrees, DeltaGdegrees (37), and T(M) for each sequence were determined. Data from this study were combined with data from previous thermodynamic data [Longfellow, C. E., Kierzek, R., and Turner, D. H. (1990) Biochemistry 29, 278-85] to develop a model that will more accurately predict the free energy of an RNA duplex containing a single nucleotide. bulge. Differences between purine and pyrimidine bulges as well as differences between Group I duplexes, those in which the bulge is not identical to either neighboring nucleotide, and Group II duplexes, those in which the bulge is identical to at least one neighboring nucleotide, were considered. The length of the duplex, non-nearest-neighbor effects, and bulge location were also examined. A model was developed which divides sequences into two groups: those with pyrimidine bulges and those with purine bulges. The proposed model for pyrimidine bulges predicts DeltaGdegrees(37,bulge) = 3.9 kcal/mol + Delta10DeltaGdegrees(37,nn) + beta, while the model for purine bulges predicts DeltaGdegrees(37,bulge) = 3.3 kcal/mol - 0.30DeltaGdegrees(37,nn) + beta, where beta has a value of 0.0 and -0.8 kcal/mol for Group I and Group II sequences, respectively, and DeltaGdegrees(37,nn) is the nearest-neighbor free energy of the base pairs surrounding the bulge. The conformation of bulge loops present in rRNA was examined. Three distinct families of structures were identified. The bulge loop was either extrahelical, intercalated, or in a "sidestep" conformation.