A type of RNA known only from bacteria and some bacteriophages that combines features of both transfer RNA (tRNA) and messenger RNA (mRNA). It contains a tRNA domain and a short coding sequence, and serves to rescue ribosomes that have stalled during protein synthesis due to a faulty mRNA molecule. The tmRNA dissociates the unfinished polypeptide from the ribosome, enabling the latter to be recycled. It also tags both the polypeptide and the defective mRNA for degradation.