User:Nathan Harris/Tus: Difference between revisions

Multiple ''Ter'' sites (''TerA''- ''TerJ'') are located in regions destined for replication termination in ''E. coli''. Tus binds specifically to these 23bp ''Ter'' sites forming a Tus-''Ter'' complex. This complex allows for the blocking of an approaching replication fork in one direction, the non-permissive face, but not from the other direction, the permissive face.  The ability to halt the replication machinery at the non-permissive face is thought to involve the inhibition of DnaB Helicase, preventing it from unwinding DNA. DnaB inhibition has been proposed to occur either through protein-protein interactions between Tus and DnaB, or by a physical block provided by Protein-DNA interactions i.e. the Tus-''Ter'' complex.  Recent models suggest a potentially combination of these two mechanisms. Evolution of this termination system has allowed for efficient replication by ''E. coli'' as it prevents any over expenditure of energy or time.  Different replication proteins have been found in other model organisms, such as RTP in ''Bacillus subtilis''.  Despite similar biological roles of RTP and Tus they have significantly different structures.   

=='''Ter sites'''==

The Tus protein binds as a monomer through several direct and indirect contacts to conserved ''Ter'' sites.  Ter sites are signified by 23 bp of consensus sequences which maintain a highly conserved C6 and 13 bp core region that interacts with Tus.  Additionally, ''Ter'' sites are arranged in groups of five located opposite to the origin of replication.  Within each group the ''Ter'' sites have a coordinated polarity of termination.   

=='''Structure of Tus protein and binding interactions with TerA'''==

The carboxy domain consists of a hydrophobic core stabilised by alpha helices and β strands (βGHNO). The L3 loop is responsible for connecting helices αVI and αVII and also contacts the minor groove of DNA.  

=='''Confirmation changes induced on Ter sites'''==

The ''Ter'' region in ''E.coli'' between bases T5 and A9 is significantly underwound upon binding with Tus. This region of DNA is altered from standard B form which is attributed to straddling of ''Ter'' by interdomain β strands (βF and βG) and the L4 connecting loop of Tus. Tus interacts with ''Ter'' in a previously undescribed manner with β strands of Tus inserting almost perpendicularly into the major groove to recognise ''Ter''. Alteration of ''Ter'' is characterised by an extended major groove and a broadened minor groove generating an overall DNA bend of 20 degrees. Overall, contacts in these regions account for increased stability of the altered DNA shape and allow recognition of the appropriate ''Ter'' site.   

=='''Mechanism of action'''==

The ability of Tus to terminate replication in ''E. coli'' in a polar manner is believed to involve the inhibition of DnaB helicase.  This is achieved either through a “locked complex” model provided by Tus-Ter interactions providing a physical block, protein-protein interactions between Tus and DnaB, or through a combination of these two effects.  

Recent models for the termination of replication in ''E. coli'' propose that when DnaB approaches the Tus-''Ter'' complex from the permissive face there are no considerable protein-protein interactions between the DnaB and Tus resulting in the dislodgement of Tus from ''Ter'' and hence allowing for the progression of the replication fork. However, when DnaB approaches the non-permissive face, significant protein-protein interactions between the DnaB and Tus prevent the dislodgement of Tus, resulting in replication termination. If for any reason this mechanism may fail, DnaB will unwind ''Ter'' until it reaches C6 which would induce the formation of a locked complex and subsequent prevention of replication fork progression.

=='''References'''==