R bodies (from ''refractile'' bodies, also R-bodies) are polymeric protein inclusions formed inside the cytoplasm of

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

. Initially discovered in

kappa particle Kappa (; uppercase Κ, lowercase κ or cursive ; , ''káppa'') is the tenth letter of the Greek alphabet, representing the voiceless velar plosive sound in Ancient and Modern Greek. In the system of Greek numerals, has a value of 20. It was d ...

s, bacterial endosymbionts of the ciliate ''

Paramecium ''Paramecium'' ( , , plural "paramecia" only when used as a Common name, vernacular name) is a genus of eukaryotic, unicellular ciliates, widespread in freshwater, brackish, and Ocean, marine environments. Paramecia are often abundant in stagna ...

'', R bodies (and genes encoding them) have since been discovered in a variety of taxa.

Morphology, assembly, and extension

At neutral pH, type 51 R bodies resemble a coil of ribbon approximately 500 nm in diameter and approximately 400 nm deep. Encoded by a single

operon In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splic ...

containing four

open reading frame In molecular biology, reading frames are defined as spans of DNA sequence between the start and stop codons. Usually, this is considered within a studied region of a prokaryotic DNA sequence, where only one of the six possible reading frames ...

s, R bodies are formed from two small structural proteins, RebA and RebB. A third protein, RebC, is required for the covalent assembly of these two structural proteins into higher-molecular weight products, visualized as a ladder on an SDS-PAGE gel. At low pH, Type 51 R bodies undergo a dramatic structural rearrangement. Much like a paper yo-yo, the ribbon extends (from the center) to form hollow tube with pointed ends that can reach up to 20μm in length. Other types of R bodies from different bacterial species vary in their size, ribbon morphology, and triggers for extension.

Function

When

s shed from a killer paramecium are ingested, R bodies extend within the acidic food vacuole of the predatory paramecium, distending and rupturing the membrane. This liberates the contents of the food vacuole into the cytoplasm of the paramecium. While feeding kappa particles to sensitive paramecium results in the death of paramecium, feeding purified R bodies or R bodies recombinantly expressed in ''E. coli'' is not toxic. Thus, R bodies are thought to function as a toxin delivery system. R bodies are also capable of rupturing ''E. coli''

spheroplast A spheroplast (or sphaeroplast in British usage) is a microbial cell from which the cell wall has been almost completely removed, as by the action of penicillin or lysozyme. According to some definitions, the term is used to describe Gram-negative ...

s, demonstrating that they can rupture membranes in a foreign context, and they can be engineered to extend at a variety of different pH levels.

References

{{Reflist Cell biology Cell anatomy Protein complexes Bacteriology Biotechnology