Tus (terminus utilization substance), also known as a ter-binding protein, is a protein that binds to terminator sequences and acts as a counter-

helicase Helicases are a class of enzymes that are vital to all organisms. Their main function is to unpack an organism's genetic material. Helicases are motor proteins that move directionally along a nucleic double helix, separating the two hybridized ...

when it comes in contact with an advancing helicase. The bound Tus protein effectively halts

DNA polymerase A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to create t ...

movement. Tus helps end

DNA replication In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all life, living organisms, acting as the most essential part of heredity, biolog ...

prokaryote A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...

s.Slonczewski, Joan, and John Watkins. Foster. Microbiology: An Evolving Science. New York: W.W. Norton &, 2009. Print. They function by binding to DNA replication terminator sequences, thus preventing the passage of replication forks. The termination efficiency is affected by the affinity of a particular protein for the terminator

sequence In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is cal ...

. In ''E. coli'' (), Tus binds to 10 closely related sites encoded in the

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

, although only 6 are likely to be involved in replication termination. Each site is 23 base pairs. The sites are called Ter sites, and are designated ''TerA'', ''TerB'', ..., ''TerG''. These binding sites are asymmetric, such that when a Tus-Ter complex (Tus protein bound to a Ter site) is encountered by a replication fork from one direction, the complex is dissociated and replication continues (permissive). But when encountered from the other direction, the Tus-Ter complex provides a much larger kinetic barrier and halts replication (non-permissive). The multiple ''Ter'' sites in the chromosome are oriented such that the two oppositely moving replication forks are both stalled in the desired termination region. ''

Bacillus subtilis ''Bacillus subtilis'' (), known also as the hay bacillus or grass bacillus, is a gram-positive, catalase-positive bacterium, found in soil and the gastrointestinal tract of ruminants, humans and marine sponges. As a member of the genus ''Bacill ...

'' utilize replication terminator protein (RTP) instead of Tus. This is a different protein family using a different structure to bind to DNA: .

Structure

The Ter protein contains two domains. The N-terminal domain is composed of an

alpha helices An alpha helix (or α-helix) is a sequence of amino acids in a protein that are twisted into a coil (a helix). The alpha helix is the most common structural arrangement in the secondary structure of proteins. It is also the most extreme type of l ...

, beta sheet, and three loops. The C-terminal domain is made of two

and one

beta sheet The beta sheet (β-sheet, also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a gene ...

. Alternatively,

CATH Cath may refer to: __NOTOC__ People * Cath Bishop (born 1971), British former rower and 2003 world champion * Cath Carroll (born 1960), British musician and music journalist * Cath Coffey (), one of the earliest members of British rap band Stereo ...

divides the structure into a big domain spanning the entire sequence and a small insertion that pops out and folds as a separate domain.

Function

A DNA replication terminus (Ter) has a role in preventing progress of the DNA

replication fork In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms, acting as the most essential part of biological inheritanc ...

. Therefore, a DNA replication terminus site-binding protein binds to this site helping to block the DNA replication fork. There are two genes controlling Ter-binding activity, named tau and Tus.

References

{{InterPro content, IPR008865 Protein families DNA replication

Structure

Function

Further reading

References