Pxr sRNA is a
regulatory RNA
RNA interference (RNAi) is a biological process in which RNA molecules are involved in sequence-specific suppression of gene expression by double-stranded RNA, through translational or transcriptional repression. Historically, RNAi was known by o ...
which downregulates genes responsible for the formation of fruiting bodies in ''
Myxococcus xanthus
''Myxococcus xanthus'' is a gram-negative, rod-shaped species of myxobacteria that exhibits various forms of self-organizing behavior in response to environmental cues. Under normal conditions with abundant food, it exists as a predatory, saproph ...
''.
Fruiting bodies are aggregations of
myxobacteria
The myxobacteria ("slime bacteria") are a group of bacteria that predominantly live in the soil and feed on insoluble organic substances. The myxobacteria have very large genomes relative to other bacteria, e.g. 9–10 million nucleotides except ...
formed when
nutrients
A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
are scarce, the fruiting bodies permit a small number of the aggregated
colony
In modern parlance, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the ''metropole, metropolit ...
to transform into stress-resistant
spore
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...
s.
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
Starvation is a severe deficiency in caloric energy intake, below the level needed to maintain an organism's life. It is the most extreme form of malnutrition. In humans, prolonged starvation can cause permanent organ damage and eventually, dea ...
. This finding implies that Pxr-S is specifically responsible for inhibiting the fruiting body development during
cell growth
Cell growth refers to an increase in the total mass of a cell, including both cytoplasmic, nuclear and organelle volume. Cell growth occurs when the overall rate of cellular biosynthesis (production of biomolecules or anabolism) is greater than ...
when nutrients are abundant.
Pxr
homologs
A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same loci where they provide points alon ...
have only been found in one other taxon, namely ''
Stigmatella aurantiaca
''Stigmatella aurantiaca'' is a member of myxobacteria, a group of gram-negative bacteria with a complex developmental life cycle.
Classification
The bacterial nature of this organism was recognized by Thaxter in 1892, who grouped it among the ...
''. Homologs were not found in any other myxobacteria (such as ''
Sorangium cellulosum
''Sorangium cellulosum'' is a soil-dwelling Gram-negative bacterium of the group myxobacteria. It is motile and shows gliding motility. Under stressful conditions this motility, as in other myxobacteria, the cells congregate to form fruiting bodi ...
'' or ''
Anaeromyxobacter dehalogenans
''Anaeromyxobacter dehalogenans'' is a species of bacteria. It is an aryl- halorespiring facultative anaerobic myxobacterium. Its cells are slender, gram-negative rods with a bright red pigmentation that exhibit gliding motility
Motility is ...
'') which suggests the Pxr RNA gene may have a recent
evolutionary origin in the sub-clade
Myxococcales
The myxobacteria ("slime bacteria") are a group of bacteria that predominantly live in the soil and feed on insoluble organic substances. The myxobacteria have very large genomes relative to other bacteria, e.g. 9–10 million nucleotides except ...
.
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.
''M. xanthus'' obligate cheat and phoenix phenotypes
Several
mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
s in the Pxr sRNA gene have been observed.
The first mutation causes an obligate cheat (OC)
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
to emerge, these bacteria exploit the fruiting bodies of ''
wild-type
The wild type (WT) is the phenotype of the typical form of a species as it occurs in nature. Originally, the wild type was conceptualized as a product of the standard "normal" allele at a locus, in contrast to that produced by a non-standard, "m ...
'' ''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
RNA interference (RNAi) is a biological process in which RNA molecules are involved in sequence-specific suppression of gene expression by double-stranded RNA, through translational or transcriptional repression. Historically, RNAi was known by o ...
-like processing elements have been
knocked out
A knockout (abbreviated to KO or K.O.) is a fight-ending, winning criterion in several full-contact combat sports, such as boxing, kickboxing, muay thai, mixed martial arts, karate, some forms of taekwondo and other sports involving strikin ...
.
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.
After this event, a new phenotype emerged via
spontaneous mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitos ...
dubbed phoenix (PX).
The PX phenotype was developmentally superior to both OC and ''wt'', it was able to sporulate
autonomous
In developmental psychology and moral, political, and bioethical philosophy, autonomy, from , ''autonomos'', from αὐτο- ''auto-'' "self" and νόμος ''nomos'', "law", hence when combined understood to mean "one who gives oneself one's ow ...
ly - without forming fruiting bodies and with high efficiency.
Two-component system operon (histidine kinase gene and a σ54 response regulator) is associated with production and processing of Pxr sRNA.
References
Further reading
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*
*
External links
* {{Rfam, id=RF01812, name=Pxr regulatory sRNA
Non-coding RNA