Pfl RNA Motif

The ''pfl'' RNA motif (now called the ZMP/ZTP riboswitch) refers to a conserved RNA structure present in some bacteria and originally discovered using bioinformatics. ''pfl'' RNAs are consistently present in genomic locations that likely correspond to the 5' untranslated regions (5' UTRs) of protein-coding genes. This arrangement in bacteria is commonly associated with cis-regulatory elements. Moreover, they are in presumed 5' UTRs of multiple non-homologous genes, suggesting that they function only in these locations. Additional evidence of ''cis''-regulatory function came from the observation that predicted rho-independent transcription terminators overlap ''pfl'' RNAs. This overlap suggests that the alternate secondary structures of ''pfl'' RNA and the transcription terminator stem-loops compete with each other, and this is a common mechanism for ''cis'' gene control in bacteria.

''pfl'' RNAs are found in a variety of phyla of bacteria, but are not found in all the species of that phylum. ''pfl'' RNAs are common among species of orders Actinomycetales and Clostridiales, the classes Alphaproteobacteria and Betaproteobacteria and the genus ''Deinococcus''. They are also found in isolated species of Bacteroidota, Chloroflexota, and Deltaproteobacteria.

Several lines of evidence led to the hypothesis that ''pfl'' RNAs function as riboswitches. First, the above evidence that ''pfl'' RNAs correspond to ''cis''-regulatory elements is consistent with most known riboswitches. Second, their relatively complex pseudoknotted secondary structure is typical of riboswitches. Finally, several nucleotide positions are highly conserved despite the large evolutionary distance between species that use ''pfl'' RNAs; this high level of conservation is often a consequence of the need to form intricate structures to specifically bind a metabolite. Experimental evidence already supported the hypothesis that ''pfl'' RNAs function as ''cis'' regulatory elements, before the ligand was confirmed to be ZTP, as well as ZMP (also called AICAR), in 2015.

The genes presumed to be regulated by ''pfl'' RNAs relate to one-carbon metabolism. Most obviously, for example, formate-tetrahydrofolate ligase synthesizes 10-formyltetrahydrofolate. The ''glyA'' and ''folD'' convert between other one-carbon adducts of tetrahydrofolate. Another gene commonly associated with ''pfl'' RNAs is ''purH'', which catalyzes the formylation of the intermediate AICAR in ''de novo'' synthesis of purines. The formyl group is taken from formyltetrahydrofolate, and purine biosynthesis is often the dominant user of formyltetrahydrofolate. In similar fashions, if less directly, most ''pfl'' RNAs are associated with genes that are directly or indirectly involved in one-carbon metabolism. It appears that the ZTP/ZMP purine derivatives can be used to regulate one-carbon metabolism by indirectly sensing a shortage of 10-formyl-tetrahydrofolate.

The atomic-resolution structure has been solved by X-ray crystallography.

References

External links

* {{Rfam, id=RF01750, name=pfl RNA

Cis-regulatory RNA elements