Pxr sRNA

Pxr sRNA is a regulatory RNA which downregulates genes responsible for the formation of fruiting bodies in Myxococcus xanthus.[1] Fruiting bodies are aggregations of myxobacteria formed when nutrients are scarce,[2] the fruiting bodies permit a small number of the aggregated colony to transform into stress-resistant spores.[3]

Pxr sRNA
Secondary structure of Pxr sRNA.
Identifiers
SymbolPxr
RfamRF01812
Other data
RNA typesRNA
Domain(s)Myxococcus xanthus, Stigmatella aurantiaca
PDB structuresPDBe

Pxr exists in two forms: Pxr-L (a long form) and Pxr-S which is shorter. The short form was found to be expressed in cells during growth but is rapidly repressed during starvation. This finding implies that Pxr-S is specifically responsible for inhibiting the fruiting body development during cell growth when nutrients are abundant.[1]

Pxr homologs have only been found in one other taxon, namely Stigmatella aurantiaca. Homologs were not found in any other myxobacteria (such as Sorangium cellulosum[4] or Anaeromyxobacter dehalogenans[5]) which suggests the Pxr RNA gene may have a recent evolutionary origin in the sub-clade Myxococcales.[1]

PxR sRNA folds into 3 steam loops. SL1 and SL 2 are highly conserved across mycobacteria and SL1 is necessary for the regulatory function. However, a conserved eight-base-pair segment of the variable SL3 is necessary for PxR accumulation and multicellular development.[6]

M. xanthus obligate cheat and phoenix phenotypes

Several mutations in the Pxr sRNA gene have been observed.[7] The first mutation causes an obligate cheat (OC) phenotype to emerge, these bacteria exploit the fruiting bodies of wild-type M. xanthus to sporulate more efficiently. This phenotype is thought to be caused by a mutation which prevents the repression of Pxr-S, thereby inhibiting the formation of fruiting bodies indefinitely. If Pxr-S is derived from Pxr-L, it may be that RNAi-like processing elements have been knocked out.[1]

In a laboratory experiment, the OC phenotype out-competed and excluded the wild type, eventually bringing about a population crash when there were not enough wild type bacteria to exploit.[7] After this event, a new phenotype emerged via spontaneous mutation dubbed phoenix (PX).[1] The PX phenotype was developmentally superior to both OC and wt, it was able to sporulate autonomously - without forming fruiting bodies and with high efficiency.[7] Two-component system operon (histidine kinase gene and a σ54 response regulator) is associated with production and processing of Pxr sRNA.[8]

References
  1. Yu YT, Yuan X, Velicer GJ (May 2010). "Adaptive evolution of an sRNA that controls Myxococcus development". Science. 328 (5981): 993. doi:10.1126/science.1187200. PMC 3027070. PMID 20489016.
  2. Kuner JM, Kaiser D (July 1982). "Fruiting body morphogenesis in submerged cultures of Myxococcus xanthus". J. Bacteriol. 151 (1): 458–461. doi:10.1128/jb.151.1.458-461.1982. PMC 220259. PMID 6806248.
  3. Wireman JW, Dworkin M (February 1977). "Developmentally induced autolysis during fruiting body formation by Myxococcus xanthus". J. Bacteriol. 129 (2): 798–802. doi:10.1128/jb.129.2.798-802.1977. PMC 235013. PMID 402359.
  4. Schneiker S, Perlova O, Kaiser O, et al. (November 2007). "Complete genome sequence of the myxobacterium Sorangium cellulosum". Nat. Biotechnol. 25 (11): 1281–1289. doi:10.1038/nbt1354. PMID 17965706.
  5. Thomas SH, Wagner RD, Arakaki AK, et al. (2008). "The mosaic genome of Anaeromyxobacter dehalogenans strain 2CP-C suggests an aerobic common ancestor to the delta-proteobacteria". PLOS ONE. 3 (5): e2103. doi:10.1371/journal.pone.0002103. PMC 2330069. PMID 18461135. open access
  6. Yu, Yuen-Tsu N.; Cooper, Elizabeth; Velicer, Gregory J. (2017-11-13). "A conserved stem of the Myxococcus xanthus sRNA Pxr controls sRNA accumulation and multicellular development". Scientific Reports. 7 (1): 15411. doi:10.1038/s41598-017-15439-w. ISSN 2045-2322. PMC 5684412. PMID 29133885.
  7. Fiegna F, Yu YT, Kadam SV, Velicer GJ (May 2006). "Evolution of an obligate social cheater to a superior cooperator". Nature. 441 (7091): 310–314. doi:10.1038/nature04677. PMID 16710413. S2CID 4371886.
  8. Yu, Yuen-Tsu N.; Kleiner, Manuel; Velicer, Gregory J. (December 1, 2016). "Spontaneous Reversions of an Evolutionary Trait Loss Reveal Regulators of a Small RNA That Controls Multicellular Development in Myxobacteria". Journal of Bacteriology. 198 (23): 3142–3151. doi:10.1128/JB.00389-16. ISSN 1098-5530. PMC 5105895. PMID 27621281.

Further reading
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