Noncanonical features and modifications on the 5′-end of bacterial sRNAs and mRNAs

Although many eukaryotic transcripts contain cap structures, it has been long thought that bacterial RNAs do not carry any special modifications on their 5′-ends. In bacteria, primary transcripts are produced by transcription initiated with a nucleoside triphosphate and are therefore triphosphorylated on 5′-ends. Some transcripts are then processed by nucleases that yield monophosphorylated RNAs for specific cellular activities. Many primary transcripts are also converted to monophosphorylated species by removal of the terminal pyrophosphate for 5′-end-dependent degradation. Recent studies surprisingly revealed an expanded repertoire of chemical groups on 5′-ends of bacterial RNAs. In addition to mono- and triphosphorylated moieties, some mRNAs and sRNAs contain cap-like structures and diphosphates on their 5′-ends. Although incorporation and removal of these groups have become better understood in recent years, the physiological significance of these modifications remain obscure. This review highlights recent studies aimed at identification and elucidation of novel modifications on the 5′-ends of bacterial RNAs and discusses possible physiological applications of the modified RNAs. This article is categorized under: RNA Turnover and Surveillance > Regulation of RNA Stability RNA Structure and Dynamics > RNA Structure, Dynamics, and Chemistry RNA Processing > Capping and 5′ End Modifications.