Recognition of RNA by triplex formation: divergent effects of pyrimidine C-5 methylation.

In DNA triple helices, methylation at C-5 of thymine or cytosine is reported to have similar stabilizing effects for both bases. Here we show, however, that methylation of the same positions in RNA triplexes has distinctly different effects than in DNA. We have previously described the use of circular triplex-forming RNA oligonucleotides to recognize RNA sequences. Here it is shown that addition of C-5 methyl groups to uracils in these compounds very significantly increases not only affinity but also sequence selectivity in binding a purine-rich RNA target, as measured by thermal denaturation with various target RNAs. Surprisingly, however, addition of C-5 methyl groups to cytosines actually decreases affinity in binding RNA, while the same substitution in DNA is thermally stabilizing. Possible sources of this divergent behavior are discussed. A synthesis of 5-methylcytidine ribonucleoside 2'-O-silyl-3'-O-phosphoramidite is also described.