φXo, Xf, φLf, φXv, and Cf are filamentous bacteriophages isolated in Taiwan. Both φXo and Xf specifi- cally infect Xanthomonas oryzae pv. oryzae, and φLf, φXv, and Cf specifically infect X. campestris pv. campestris, X. campestris pv. vesicatoria, and X. campestris pv. citri, respectively. In this study, the φXo gene III (gIII) encod- ing the adsorption protein (pIII) was cloned by probing with the φLf gIII. Sequence analysis revealed that the φXo gIII is 1,023-nt long and able to encode a pre-protein of 340 aa (35,337 Da), with structural features typical of filamentous phage adsorption proteins: an N-terminal signal sequence (18 aa), a central region (90 aa) containing 38 glycine, 29 aspartic acid and 19 histidine residues, and a C-terminal membrane-anchoring domain (17 aa). The φXo pIII purified from the phage particles migrated as a 42-kDa band in SDS-polyacrylamide gel, which is substantially larger than that deduced from the nucleotide sequence, presumably due to the presence of the long stretch of charged residues in the central region. The φXo pIII could cross-react with the antiserum specific to φLf pIII, which also cross-reacts with φXv pIII. Like the situations in φLf and φXv, the pIIIs of φLf and φXo are also interchangeable. The gIII and the flanking regions of φLf, φXv and φXo are highly homologous and similar in size, but φXo has a genome (7.6 kb) larger than that of φLf (6.0 kb) and φXv (6.4 kb), suggesting that φXo is able to accommodate more genes and/or have longer intergenic regions in the remaining part of the genome. Difference in sizes between the pIIIs indicates that φXo and Xf, which has a predicted pre-pIII of 488 aa (51,036 Da), are distinct phages.

φXo, Xf, φLf, φXv, and Cf are filamentous bacteriophages isolated in Taiwan. Both φXo and Xf specifi- cally infect Xanthomonas oryzae pv. oryzae, and φLf, φXv, and Cf specifically infect X. campestris pv. campestris, X. campestris pv. vesicatoria, and X. campestris pv. citri, respectively. In this study, the φXo gene III (gIII) encod- ing the adsorption protein (pIII) was cloned by probing with the φLf gIII. Sequence analysis revealed that the φXo gIII is 1,023-nt long and able to encode a pre-protein of 340 aa (35,337 Da), with structural features typical of filamentous phage adsorption proteins: an N-terminal signal sequence (18 aa), a central region (90 aa) containing 38 glycine, 29 aspartic acid and 19 histidine residues, and a C-terminal membrane-anchoring domain (17 aa). The φXo pIII purified from the phage particles migrated as a 42-kDa band in SDS-polyacrylamide gel, which is substantially larger than that deduced from the nucleotide sequence, presumably due to the presence of the long stretch of charged residues in the central region. The φXo pIII could cross-react with the antiserum specific to φLf pIII, which also cross-reacts with φXv pIII. Like the situations in φLf and φXv, the pIIIs of φLf and φXo are also interchangeable. The gIII and the flanking regions of φLf, φXv and φXo are highly homologous and similar in size, but φXo has a genome (7.6 kb) larger than that of φLf (6.0 kb) and φXv (6.4 kb), suggesting that φXo is able to accommodate more genes and/or have longer intergenic regions in the remaining part of the genome. Difference in sizes between the pIIIs indicates that φXo and Xf, which has a predicted pre-pIII of 488 aa (51,036 Da), are distinct phages.

Gene III ( gIII ) of φLf, a filamentous phage specifically infecting Xanthomonas campestris pv. campestris, was previously shown to encode a virion-associated protein (pIII) required for phage adsorption. In this study, the transcription start site for the gene and the N-terminal sequence of the protein were determined, resulting in the revision of the translation initiation site from the one previously predicted for this gene. For comparative study, the gIII of φXv, a filamentous phage specifically infecting X. campestris pv. vesicatoria, was cloned and sequenced. The deduced amino acid sequences of these two pIIIs exhibit a high degree of identity in their C-terminal halves and possess the structural features typical of the adsorption proteins of filamentous phages: a signal sequence in the N terminus, a long glycine-rich region near the center, and a hydrophobic membrane anchorage domain in the C terminus. The regions between gIII and the upstream gVIII , 128 nucleotides in both phages, are larger than those of other filamentous phages. A hybrid phage of φXv, consisting of the φLf pIII and all the other components derived from φXv, was able to infect X. campestris pv. campestris but not X. campestris pv. vesicatoria, indicating that gIII is the gene specifying host specificity and demonstrating the interchangeability of the pIIIs.