A dynamic programming algorithm for prediction of RNA secondary structure has been revised to accommodate folding constraints determined by chemical modification and to include free energy increments for coaxial stacking of helices when they are either adjacent or separated by a single mismatch. Furthermore, free energy parameters are revised to account for recent experimental results for terminal mismatches and hairpin, bulge, internal, and multibranch loops. To demonstrate the applicability of this method, in vivo modification was performed on 5S rRNA in both Escherichia coli and Candida albicans with 1-cyclohexyl-3-(2-morpholinoethyl) carbodiimide metho-p-toluene sulfonate, dimethyl sulfate, and kethoxal. The percentage of known base pairs in the predicted structure increased from 26.3% to 86.8% for the E. coli sequence by using modification constraints. For C. albicans, the accuracy remained 87.5% both with and without modification data. On average, for these sequences and a set of 14 sequences with known secondary structure and chemical modification data taken from the literature, accuracy improves from 67% to 76%. This enhancement primarily reflects improvement for three sequences that are predicted with <40% accuracy on the basis of energetics alone. For these sequences, inclusion of chemical modification constraints improves the average accuracy from 28% to 78%. For the 11 sequences with <6% pseudoknotted base pairs, structures predicted with constraints from chemical modification contain on average 84% of known canonical base pairs. R ecent discoveries have shown that RNA plays a larger role in biology than previously realized, e.g., in posttranscriptional regulation (1), development (2, 3), immunity (4, 5), and peptide bond formation (6, 7). It is necessary to determine the native structures of RNAs to understand their mechanisms of action, and determining secondary structure is a crucial step in this process.RNA secondary structure can be predicted by free energy minimization with nearest neighbor parameters to evaluate stability (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). Previous studies demonstrated that nuclease cleavage data can be used to refine structure prediction and improve accuracy (8, 11). A predicted secondary structure can guide further experiments or comparative sequence analysis (19) and also aid in the design of RNA molecules (20,21).Chemical modification is a technique that reveals solvent accessible nucleotides (22). The nucleotides accessible to 1-cyclohexyl-3-(2-morpholinoethyl) carbodiimide metho-ptoluene sulfonate, dimethyl sulfate, and kethoxal are unpaired, in A-U or G-C pairs at helix ends, in G-U pairs anywhere, or adjacent to G-U pairs. This limited specificity differs from that observed with nucleases, and an algorithm allowing constraints from such chemical modification has not been reported. Chemical modification is used extensively to test hypothesized RNA secondary structures (19,(23)(24)(25)(26)(27)(28). Chemical modification can also be used to deduce possible tertia...