title: The Agrobacterium tumefaciens virB4 gene product is an essential virulence protein requiring an intact nucleoside triphosphate-binding domain.
abstract: Products of the approximately 9.5-kb virB operon are proposed to direct the export of T-DNA/protein complexes across the Agrobacterium tumefaciens envelope en route to plant cells. The presence of conserved nucleoside triphosphate (NTP)-binding domains in VirB4 and VirB11 suggests that one or both proteins couple energy, via NTP hydrolysis, to T-complex transport. To assess the importance of VirB4 for virulence, a nonpolar virB4 null mutation was introduced into the pTiA6NC plasmid of strain A348. The 2.37-kb virB4 coding sequence was deleted precisely by oligonucleotide-directed mutagenesis in vitro. The resulting delta virB4 mutation was exchanged for the wild-type allele by two sequential recombination events with the counterselectable Bacillus subtilis sacB gene. Two derivatives, A348 delta B4.4 and A348 delta B4.5, sustained a nonpolar deletion of the wild-type virB4 allele, as judged by Southern blot hybridization and immunoblot analyses with antibodies specific for VirB4, VirB5, VirB10, and VirB11. Transcription of wild-type virB4 from the lac promoter restored virulence to the nonpolar null mutants on a variety of dicotyledonous species, establishing virB4 as an essential virulence gene. A substitution of glutamine for Lys-439 and a deletion of Gly-438, Lys-439, and Thr-440 within the glycine-rich NTP-binding domain (Gly-Pro-Iso-Gly-Arg-Gly-Lys-Thr) abolished complementation of A348 delta B4.4 or A348 delta B4.5, demonstrating that an intact NTP-binding domain is critical for VirB4 function. Merodiploids expressing both the mutant and wild-type virB4 alleles exhibited lower virulence than A348, suggesting that VirB4, a cytoplasmic membrane protein, may contribute as a homo- or heteromultimer to A. tumefaciens virulence.
authors.affiliation: Department of Microbiology and Molecular Genetics, University of Texas Health Science Center, Houston 77030.
authors.forename: B R
authors.initials: BR
authors.lastname: Berger
authors.forename: P J
authors.initials: PJ
authors.lastname: Christie
mesh_terms: D016960:Agrobacterium tumefaciens / Q000235:genetics* / Q000472:pathogenicity; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D003001:Cloning, Molecular; D005798:Genes, Bacterial; D005816:Genetic Complementation Test; D015151:Immunoblotting; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009711:Nucleotides / Q000378:metabolism*; D010957:Plasmids; D014774:Virulence / Q000235:genetics; D037521:Virulence Factors
chemical_list: D001426:Bacterial Proteins; D009711:Nucleotides; D037521:Virulence Factors