Journal
STRUCTURE
Volume 11, Issue 11, Pages 1369-1379Publisher
CELL PRESS
DOI: 10.1016/j.str.2003.10.001
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Conjugative plasmid transfer between bacteria disseminates antibiotic resistance and diversifies prokaryotic genomes. Relaxases, proteins essential for conjugation, cleave one plasmid strand sequence specifically prior to transfer. Cleavage occurs through a Mg2+-dependent transesterification involving a tyrosyl hydroxyl and a DNA phosphate. The structure of the F plasmid Tral relaxase domain, described here, is a five-strand beta sheet flanked by alpha helices. The protein resembles replication initiator protein AAV-5 Rep but is circularly permuted, yielding a different topology. The beta sheet forms a binding cleft lined with neutral, nonaromatic residues, unlike most single-stranded DNA binding proteins which use aromatic and charged residues. The cleft contains depressions, suggesting base recognition occurs in a knob-into-hole fashion. Unlike most nucleases, three histidines but no acidic residues coordinate a Mg2+ located near the catalytic tyrosine. The full positive charge on the Mg2+ and the architecture of the active site suggest multiple roles for Mg2+ in DNA cleavage.
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