Rop is a small homodimeric four-helix bundle protein formed by the antiparallel interaction of two helix-turn-helix monomers. The protein is expressed 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...

as a mechanism for regulating the gene copy number

Gene copy number

Copy-number variations —a form of structural variation—are alterations of the DNA of a genome that results in the cell having an abnormal number of copies of one or more sections of the DNA. CNVs correspond to relatively large regions of the genome that have been deleted or duplicated on certain...

s of plasmid

Plasmid

In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...

s. The Rop protein's structure has been solved to high resolution. Due to its small size and known structure, Rop has been used in protein design

Protein design

Protein design is the design of new protein molecules, either from scratch or by making calculated variations on a known structure. The use of rational design techniques for proteins is a major aspect of protein engineering....

work to rearrange its helical topology and reengineer its loop regions. In general, the four-helix bundle has been extensively used in de novo protein design work as a simple model to understand the relationship between amino acid sequence and structure.