Editing RNA world (section)

=== RNA as a regulator ===
{{Main|Riboswitch}}
Riboswitches have been found to act as regulators of gene expression, particularly in bacteria, but also in plants and [[archaea]]. Riboswitches alter their [[secondary structure]] in response to the binding of a [[metabolite]]. Riboswitch classes have highly conserved [[aptamer]] domains, even among diverse organisms. When a target metabolite is bound to this aptamer, conformational changes occur, modulating the expression of genes carried by mRNA. These changes occur in an expression platform, located downstream from the aptamer.<ref>{{Cite journal |last1=Winkler |first1=Wade C. |last2=Breaker |first2=Ronald R. |date=2005-10-01 |title=Regulation of Bacterial Gene Expression by Riboswitches |url=http://dx.doi.org/10.1146/annurev.micro.59.030804.121336 |journal=Annual Review of Microbiology |volume=59 |issue=1 |pages=487–517 |doi=10.1146/annurev.micro.59.030804.121336 |pmid=16153177 |issn=0066-4227}}</ref> This change in structure can result in the formation or disruption of a [[Terminator (genetics)|terminator]], truncating or permitting transcription respectively.<ref>{{cite journal | vauthors = Nudler E, Mironov AS | title = The riboswitch control of bacterial metabolism | journal = Trends in Biochemical Sciences | volume = 29 | issue = 1 | pages = 11–17 | date = January 2004 | pmid = 14729327 | doi = 10.1016/j.tibs.2003.11.004 }}</ref> Alternatively, riboswitches may bind or occlude the [[Shine–Dalgarno sequence]], affecting translation.<ref>{{cite journal | vauthors = Tucker BJ, Breaker RR | title = Riboswitches as versatile gene control elements | journal = Current Opinion in Structural Biology | volume = 15 | issue = 3 | pages = 342–348 | date = June 2005 | pmid = 15919195 | doi = 10.1016/j.sbi.2005.05.003 }}</ref> It has been suggested that these originated in an RNA-based world.<ref name="pmid18778966">{{cite journal | vauthors = Bocobza SE, Aharoni A | title = Switching the light on plant riboswitches | journal = Trends in Plant Science | volume = 13 | issue = 10 | pages = 526–533 | date = October 2008 | pmid = 18778966 | doi = 10.1016/j.tplants.2008.07.004 | bibcode = 2008TPS....13..526B }}</ref> In addition, [[RNA thermometer]]s regulate gene expression in response to temperature changes.<ref name="Nar06">{{cite journal | vauthors = Narberhaus F, Waldminghaus T, Chowdhury S | title = RNA thermometers | journal = FEMS Microbiology Reviews | volume = 30 | issue = 1 | pages = 3–16 | date = January 2006 | pmid = 16438677 | doi = 10.1111/j.1574-6976.2005.004.x | doi-access = free }}</ref>