Topo IV is the primary decatenase which separates newly replicated chromosomes prior to cell division, without this decatenase, large nucleoids would accumulate in the midcell during the late stage of replication. (1)
Topo IV is encoded by parC and parE (2,3,4), parC and parE are located at map position 68.1 min and 68.3 min respectively on E. coli chromosome (5,6,7). Tope IV is a heterotetramer in the form of ParC2ParE2(8), ParC is an 83.7kDa protein with 752aa(5,6,7) and ParE is a 70,2kDa protein with 630aa(5,6,7) ParC is responsible for DNA binding and the cleavage-religation activity. (8,9,10), while ParE may involved in ATP binding and hydrolysis due to its homologous to GyrB. (11).
The most important activity of topo IV in vivo is decatenation, which occurs processively. If there is no topo IV, all most all newly replicated DNA is catenated.
Two-Gate Mechanism
Type II topoisomerases break one duplex strand of DNA which is named as ‘G-segment’ because of its gate-like character, and pass one duplex strand of DNA which is named as ‘T-segment’ because of its transport-like character. (12,13) The subunit of topo IV ParC binds to DNA by forming a covalent phosphotyrosine bond between an active site of tyrosine and phosphate backbone, the segment of DNA that binds to ParC is G-segment. ParE subunits form dimer by comsuming ATP, then it clamp the T-segment making the DNA through the G-segment which also within the enzyme. The T-segment finally get out of the enzyme through another protein gate - ‘C-gate’ which is formed by the ParC subunits and is on the other side of the enzyme. (12,13)
References:
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13. J.C.Wang, R.I. Gumport, K.Javaherian, et al in: B. Alberts (Ed.), Mechanistic Studies of DNA replication and Genetic Recombination, Academic Press, New York, 1980, pp. 769-784.