>TIGR01051 topA_bact DNA topoisomerase I; InterPro: IPR005733 DNA topoisomerases regulate the number of topological links between two DNA strands (i.e. change the number of superhelical turns) by catalysing transient single- or double-strand breaks, crossing the strands through one another, then resealing the breaks. These enzymes have several functions: to remove DNA supercoils during transcription and DNA replication; for strand breakage during recombination; for chromosome condensation; and to disentangle intertwined DNA during mitosis , . DNA topoisomerases are divided into two classes: type I enzymes (5.99.1.2 from EC; topoisomerases I, III and V) break single-strand DNA, and type II enzymes (5.99.1.3 from EC; topoisomerases II, IV and VI) break double-strand DNA . Type I topoisomerases are ATP-independent enzymes (except for reverse gyrase), and can be subdivided according to their structure and reaction mechanisms: type IA (bacterial and archaeal topoisomerase I, topoisomerase III and reverse gyrase) and type IB (eukaryotic topoisomerase I and topoisomerase V). These enzymes are primarily responsible for relaxing positively and/or negatively supercoiled DNA, except for reverse gyrase, which can introduce positive supercoils into DNA. This entry describes topoisomerase I from bacteria, which is more closely related to archaeal than to eukaryotic topoisomerase I . Topoisomerase I is the major enzyme for relaxing negatively supercoiled DNA, and its presence is balanced by reverse gyrase, which can introduce negative supercoils. Prokaryotic topoisomerase I folds in an unusual way to give 4 distinct domains, enclosing a hole large enough to accommodate a double-stranded DNA segment. A tyrosine at the active site, which lies at the interface of 2 domains, is involved in transient breakage of a DNA strand, and the formation of a covalent protein-DNA intermediate through a 5-phosphotyrosine linkage. The structure reveals a plausible mechanism by which this and related enzymes could catalyse the passage of one DNA strand through a transient break in another strand . Topoisomerase I require Mg2+ as a cofactor for catalysis to take place. More information about this protein can be found at Protein of the Month: DNA Topoisomerase .; GO: 0003677 DNA binding, 0003917 DNA topoisomerase type I activity, 0006265 DNA topological change, 0006268 DNA unwinding during replication, 0005694 chromosome.

Probab=93.97  E-value=0.089  Score=33.40  Aligned_cols=67  Identities=24%  Similarity=0.287  Sum_probs=51.6

Q ss_pred             HHHHCCEEEEEEEHHHHCCCCCCCCHHHCCHHHHHHHHCCCCCCEEEEEECCCCCHHHHHHHHHHHHCCCCCE----EEE
Q ss_conf             9751740134210121002000268111288999998515785549999469978689999999982017980----887
Q gi|254780762|r   97 LERSKAVNALYHVLGGSLSPLDRIGPEDIGIQSLIERIEVKKIRELIFAISATIEGQTTAHYIMDKLKGIDVK----ITR  172 (201)
Q Consensus        97 IE~t~~y~G~YhVLgG~ispl~g~~p~~l~i~~L~~ri~~~~i~EVIlA~~~t~EGe~Ta~yi~~~lk~~~ik----itr  172 (201)
T Consensus        60 ~e~~~qF~P~Y~~~~~K----------~~~V~~L~~la--kkA~~~yLAtD~DREGEAIaWHl~~~l~~~~~~~ddf~kR  127 (688)
T TIGR01051        60 IEKNKQFEPEYVVSKGK----------KKVVKELKTLA--KKADEVYLATDPDREGEAIAWHLAELLKPKLKVKDDFYKR  127 (688)
T ss_pred             CCCCCCCCCCEEECCCH----------HHHHHHHHHHH--HHCCEEEECCCCCCCHHHHHHHHHHHHCCCCCCCCCCCCC
T ss_conf             21378416504354614----------47899999998--7316156404888540258999999807546775643065


Q ss_pred             EEC
Q ss_conf             414
Q gi|254780762|r  173 LAY  175 (201)
Q Consensus       173 la~  175 (201)
T Consensus       128 ~VF  130 (688)
T TIGR01051       128 IVF  130 (688)
T ss_pred             EEC
T ss_conf             002