Prokaryotic transcription regulation by the nascent RNA elements

Transcription regulation by cis-acting elements such as DNA and RNA has not been investigated much compared to that of trans-acting elements like transcription factors because most cis-elements are much larger and more flexible than protein factors. Consequently, it was challenging to recapitulate the function of cis-elements in a reduced system for in vitro assays. However, the recent cryo-electron microscopy (cryo-EM) made it possible to study the effect of nascent RNA elements to the transcription in combination with biochemical experiments as cryo-EM does not require crystallization and tolerates heterogeneity to an extent. In this review, we briefly described the current model on prokaryotic transcriptional pausing based on the crystal and cryo-EM structures of RNA polymerases in different contexts, including an RNA hairpin pause. We then introduced two other nascent RNA elements that modulate transcription - preQ1 riboswitch and HK022 put RNA. Understanding the function of the RNA elements to transcription will deepen our understanding of the fundamental mechanism transcription and provide the structural basis for drug discovery as well as bioresearch tool development.