5'-sulfhydryl-modified RNA: initiator synthesis, in vitro transcription, and enzymatic incorporation.

The detailed syntheses of the sulfhydryl-modified guanosine monophosphates 5'-deoxy-5'-thioguanosine-5'-monophosphorothioate (GSMP), O-[omega-sulfhydryl-tetra(ethylene glycol)]-O-(5'-guanosine) monophosphate (5'-HS-PEG4-GMP), and O-[omega-sulfhydryl-di(ethylene glycol)]-O-(5'-guanosine) monophosphate (5'-HS-PEG2-GMP) are reported. Transcription reactions employing GSMP, 5'-HS-PEG4-GMP, or 5'-HS-PEG2-GMP as the initiator nucleotide for T7 RNA polymerase introduce a thiol group at the 5'-end of RNA. The efficiency of thiol incorporation at the 5'-terminus of modified RNA compounds was assayed with three different thiol-reactive biotinylated reagents followed by streptavidin gel-shift methods. The transcription efficiency with various ratios of GTP to 5'-HS-PEG2-GMP was explored by reaction with a sulfhydryl-reactive maleimide-conjugated protein. This is an efficient method to incorporate enzymatically a thiol group into the 5'-end of RNA.