The F-Plasmid TraI Protein Contains Three Functional Domains Required for Conjugative DNA Strand Transfer

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Abstract
The F-plasmid-encoded TraI protein, also known as DNA helicase I, is a bifunctional protein required for conjugative DNA transfer. The enzyme catalyzes two distinct but functionally related reactions required for the DNA processing events associated with conjugation: the site- and strand-specific transesterification (relaxase) reaction that provides the nick required to initiate strand transfer and a processive 5′-to-3′ helicase reaction that provides the motive force for strand transfer. Previous studies have identified the relaxase domain, which encompasses the first ∼310 amino acids of the protein. The helicase-associated motifs lie between amino acids 990 and 1450. The function of the region between amino acids 310 and 990 and the region from amino acid 1450 to the C-terminal end is unknown. A protein lacking the C-terminal 252 amino acids (TraIΔ252) was constructed and shown to have essentially wild-type levels of transesterase and helicase activity. In addition, the protein was capable of a functional interaction with other components of the minimal relaxosome. However, TraIΔ252 was not able to support conjugative DNA transfer in genetic complementation experiments. We conclude that TraIΔ252 lacks an essential C-terminal domain that is required for DNA transfer. We speculate this domain may be involved in essential protein-protein interactions with other components of the DNA transfer machinery.