Regulatory roles of small RNAs in prokaryotes: parallels and contrast with eukaryotic miRNA

Small non-coding RNAs continue to be identified that regulate the processes of translation and transcription in prokaryotes. A variety of regulatory mechanisms have been characterized by these regulatory RNAs that occur by complementary base pairing between the small regulatory RNA (sreRNA) and a target mRNA, including transcription attenuation, translation inhibition, translation activation, and mRNA protection. Here, we discuss the description of these mechanisms, and the key components that contribute to the interactions between the sreRNA and the target mRNA. Additionally, we classify sreRNAs into categories based on their origins. Antisense RNA (asRNA) is defined strictly as cis-encoded, trans-acting regulatory RNA, while small RNA (sRNA) is strictly defined as trans-encoded, trans-acting regulatory RNA. Although both RNAs bind the target mRNA by Watson-Crick base pairing to the complementary sequence of mRNA, sRNA binding typically requires the presence of a chaperone protein, is only partially complementary to the target mRNA, and often targets multiple mRNAs. Therefore, we characterize the mechanism of sRNA as similar to the well characterized eukaryotic miRNA and discuss the parallels and differences between the two. The binding of asRNA to its target mRNA, by contrast, is typically independent of a chaperone protein, is completely or almost completely complementary to the mRNA target sequence and targets only a single mRNA. While this categorization is likely to evolve as the research identifies more relevant and distinguishing characteristics to classify sreRNAs, the classification used here of prokaryotic sreRNAs should lead to a more refined approach to the discussion and investigation of regulatory RNAs until then.