Pxr sRNA is a regulatory RNA which downregulates genes responsible for the formation of fruiting bodies in Myxococcus xanthus

Myxococcus xanthus

Myxococcus xanthus colonies exist as a self-organized, predatory, saprotrophic, single-species biofilm called a swarm. Myxococcus xanthus, which can be found almost ubiquitously in soil, are thin rod shaped, gram-negative cells that exhibit self-organizing behavior as a response to environmental...

. Fruiting bodies are aggregations of myxobacteria

Myxobacteria

The myxobacteria are a group of bacteria that predominantly live in the soil. The myxobacteria have very large genomes, relative to other bacteria, e.g. 9-10 million nucleotides. Sorangium cellulosum has the largest known bacterial genome, at 13.0 million nucleotides...

 formed when nutrients are scarce, the fruiting bodies permit a small number of the aggregated colony to transform into stress-resistanct spores

Spore

In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...

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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

Repressor

In molecular genetics, a repressor is a DNA-binding protein that regulates the expression of one or more genes by binding to the operator and blocking the attachment of RNA polymerase to the promoter, thus preventing transcription of the genes. This blocking of expression is called...

 during starvation

Starvation

Starvation is a severe deficiency in caloric energy, nutrient and vitamin intake. It is the most extreme form of malnutrition. In humans, prolonged starvation can cause permanent organ damage and eventually, death...

. This finding implies that Pxr-S is specifically responsible for inhibiting the fruiting body development during cell growth

Cell growth

The term cell growth is used in the contexts of cell development and cell division . When used in the context of cell division, it refers to growth of cell populations, where one cell grows and divides to produce two "daughter cells"...

 when nutrients are abundant.

Pxr homologs

Homology (biology)

Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 have only been found in one other taxon, namely Stigmatella aurantiaca. Homologs were not foung in any other myoxbateria (such as Sorangium cellulosum

Sorangium cellulosum

Sorangium cellulosum is a soil-dwelling Gram-negative bacterium of the group myxobacteria. It is motile and shows gliding motility. It has an unusually-large genome of 13,033,779 base pairs, making it the largest bacterial genome sequenced to date....

or Anaeromyxobacter dehalogenans) which suggests the Pxr RNA gene may have a recent evolutionary origin

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 in the sub-clade Myxococcales.

M. Xanthus obligate cheat and phoenix phenotypes

Several mutations

Mutation

In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

 in the Pxr sRNA gene have been observed. The first mutation causes an obligate cheat (OC) phenotype

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

 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

RNAI

RNAI is a non-coding RNA that is an antisense repressor of the replication of some E. coli plasmids, including ColE1. Plasmid replication is usually initiated by RNAII, which acts as a primer by binding to its template DNA. The complementary RNAI binds RNAII prohibiting it from its initiation role...

-like processing elements have been knocked out

Gene knockout

A gene knockout is a genetic technique in which one of an organism's genes is made inoperative . Also known as knockout organisms or simply knockouts, they are used in learning about a gene that has been sequenced, but which has an unknown or incompletely known function...

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In a laboratory experiment, the OC phenotype out-competed

Competitive exclusion principle

In ecology, the competitive exclusion principle, sometimes referred to as Gause's law of competitive exclusion or just Gause's law, is a proposition which states that two species competing for the same resources cannot coexist if other ecological factors are constant...

 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 dubbed phoenix (PX). 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.