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Example sentences for: tetracysteine
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The hyperthermophilic topoisomerase I from Thermotoga maritima has been shown to coordinate one Zn(II) with a unique tetracysteine motif Cys-X-Cys-X 16 -Cys-X-Cys but Zn(II) binding is not required for relaxation activity [ 33 ] . The sequence of this unique tetracysteine motifs is somewhat different from those present in other type IA topoisomerases in that the other tetracysteine motifs always had at least two amino acids separating the pairs of cysteines (Cys-X 2-11 -Cys), instead of just one amino acid (Cys-X-Cys) in T. maritima topoisomerase I [ 33 ] . Therefore the structure and function of the single Zn(II) binding motif in T. maritima may differ from the multiple Zn(II) binding motifs in E. coli topoisomerase I. Direct interaction between DNA and the T. maritima Zn(II) binding motif has not been demonstrated.
The three tetracysteine motifs do not form a stably folded structure on its own, but when combined with the 14 kDa C-terminal domain, forms a stably folded 268 amino acid DNA binding domain (ZD domain) that has higher affinity for single-stranded DNA than the 121 amino acid 14 kDa C-terminal region by itself [ 6 ] . Recent sequence and structural analysis suggests that the 14 kDa domain is evolutionarily related to the three tetracysteine motifs and belongs to the zinc ribbon family [ 7 ] . The ZD domain in E. coli topoisomerase I probably evolved from a domain that binds five Zn(II) originally.
It also has high affinity for binding to single-stranded DNA on its own when separated from the three tetracysteine motifs [ 36 ] . Based on the structural and DNA-binding properties of the E. coli topoisomerase I 14 kDa domain, one can conclude that it is possible for a subdomain in topoisomerase I to lose the Zn(II) binding cysteines during evolution and still maintains the Zn-ribbon structure and single-strand DNA binding properties [ 7 ] .
The number of potential Zn(II) binding cysteine motifs range from none in S. cerevisiae DNA topoisomerase III to four highly conserved tetracysteine motifs in the beta family of the topoisomerase III enzymes [ 38 ] . The Zn(II) domain formed by these tetracysteine motifs may be required for interaction with single-strand DNA in removal of hypernegative supercoils [ 39 ] or disruption of early recombination intermediates between inappropriately paired DNA molecules [ 40 ] .
There is also another possible explanation for the varied number of tetracysteine motifs and requirement of Zn(II) for relaxation activity found in different type IA topoisomerases.
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