Steric effects in RNA interference: probing the influence of nucleobase size and shape.

Nonpolar nucleosides with varying size and shape have been used to study the hydrogen-bonding stabilization and steric effects on RNA interference. The uracil and adenine residues of siRNA guide strands have been replaced by nonpolar isosteres of uracil and adenine and by steric variants. RNAi experiments targeting Renilla luciferase mRNA have shown close correlation between siRNA thermal stability and gene suppression. Interestingly, siRNA modified at position 7 on the guide strand does not follow this correlation, having substantial RNAi activity despite low thermal stability. Sequence-selectivity studies were carried out at this position with mutated target mRNAs and nucleobase analogues with varied size (2,4-difluoro- and 2,4-dichlorobenzene) and different shape (2,3-dichlorobenzene, 4-methylbenzimidazole). The results point out the importance of nucleobase shape and steric effects in RNA interference.