The IbpB thermometer is an RNA thermometer

RNA thermometer

An RNA thermometer is a temperature-sensitive non-coding RNA molecule which regulates gene expression. RNA thermometers often regulate genes required during either a heat shock or cold shock response, but have been implicated in other regulatory roles such as in pathogenicity and starvation.In...

 element found in the ibpAB operon

Operon

In genetics, an operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo trans-splicing to create...

. The operon contains two heat-shock genes, encoding inclusion body binding proteins A and B (IbpA/B), and is the most drastically upregulated operon under heat-shock in Escherichia coli

Escherichia coli

Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...

.

IbpA is regulated by a ROSE element

Repression of heat shock gene expression (ROSE) element

The repression of heat shock gene expression element is an RNA element found in the 5' UTR of some heat shock protein's mRNAs. The ROSE element is an RNA thermometer that negatively regulates heat shock gene expression. The secondary structure is thought to be altered by temperature, thus it is an...

 found in its 5' UTR, while IbpB has its own heat-sensitive cis-regulatory element

Cis-regulatory element

A cis-regulatory element or cis-element is a region of DNA or RNA that regulates the expression of genes located on that same molecule of DNA . This term is constructed from the Latin word cis, which means "on the same side as". These cis-regulatory elements are often binding sites for one or...

. The activity of this thermoregulator was confirmed in vitro

In vitro

In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...

but was not found in vivo

In vivo

In vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...

, suggesting more complicated operon regulation exists in bacterial cells.

IbpB protein

The IbpB protein, whose expression is regulated by the IbpB thermometer, is 48% identical to IbpA (at the level of amino acid

Amino acid

Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 sequence) yet fulfils a different role in heat shock. When IbpB is absent, IbpA protein will form long fibril

Fibril

Fibril is a fine fiber approximately 1 nm in diameter.Cytoplasmic fibrils are observed on the protoplasmic cylinders found in most spirochetal species, although no function of the cytoplasmic fibrils has been ascribed....

s which is unusual for a heat shock protein

Heat shock protein

Heat shock proteins are a class of functionally related proteins involved in the folding and unfolding of other proteins. Their expression is increased when cells are exposed to elevated temperatures or other stress. This increase in expression is transcriptionally regulated...

; IbpB, acting as a co-chaperone

Co-chaperone

Co-chaperones are proteins that assist chaperones in protein folding and other functions.-List of Co-chaperones:*Aha1*auxilin*BAG1*CAIR-1/Bag-3*CDC37/p50*Chp1*Cysteine string protein *Cyp40*Djp1*DnaJ*E3/E4-ubiquitin ligase*FKBP52*GAK*GroES...

, inhibits IbpA from forming this structure.

Under heat shock, IbpB protein dissociates to give two smaller subunits and also rearranges its tertiary structure

Tertiary structure

In biochemistry and molecular biology, the tertiary structure of a protein or any other macromolecule is its three-dimensional structure, as defined by the atomic coordinates.-Relationship to primary structure:...

. This "remarkable conformational transformation" is thought to be essential for IbpB to act as a co-chaperone with IbpA under heat shock.

IbpB has been found to retain active for a significant time after a heat shock stimulus has been removed.