Sequence-specific detection of MicroRNAs by signalamplifying ribozymes

The sequence-specific and sensitive detection of nucleic acids is becoming increasingly important in applications ranging from basic research to diagnostics and genomics. While indirect detection based on the polymerase chain reaction (PCR), transcription-med-iated amplification (TMA), or rolling circle amplification is suffi-cient for many of these purposes, only few methods, most com-monly in situ hybridization with radiolabeled or fluorescent probes, are available for the direct detection of a nucleic acid sequence in cells, tissues, or entire organisms. The detection and quantification of small amounts of nucleic acids under these conditions often remains challenging. For example, it would be extremely useful to be able to directly detect and quantify small functional RNA sequences such as microRNAs (miRNAs) or riboswitches, that regulate gene expression,1 as this might enable a direct profiling of their expression loci inside a cell, tissue, or organism