PPROJECT SUMMARY (See instructions): mRNA in cells contains base modifications, strategically (and dynamically) placed at specific position in the RNA. For fundamental research, RNA containing site-specific modifications can be generated by chemical synthesis, but only at very short lengths. The proposed research aims to build on recent advances in co-immobilized flow transcription to develop an enzymatic system to incorporate (multiple and different) modified bases site-specifically. Such a system would enable more detailed studies of translation and alternative splicing, for example, where base modifications are known to play key roles. Synthesis of RNA with position-specific substitutions will also facilitate studies of RNA structure and dynamics, where fluorescent probes for FRET/environment studies or stable isotopes for NMR measurements are essential. A better understanding of the roles of specific base modifications will ultimately inform future RNA therapeutics.