Filamentous bacteriophages have very strict host specificities. Experi
ments were performed to investigate whether the A protein of the filam
entous phage Cf, which infects Xanthomonas campestris pv, citri but no
t X. campestris pv. oryzae, is involved in determining Cf's host speci
ficity. Tile gene encoding the A protein of Cf mas cloned and expresse
d in X: campestris pv. citri, The genomic DNA of another filamentous b
acteriophage, Xf, which infects X. campestris is pv. oryzae but not X:
campestris pv. citri, was then introduced by electroporation into X.
campestris pv. citri that had expressed thf A protein of Cf. The proge
ny phages thus produced were able to infect both X, campestris pv, ory
zae and X. campestris is pv, citri, indicating that the A protein of C
f mas incorporated into the viral particles of Xf and conferred upon X
f the ability to infect the host of Cf, Inactivation of the A protein
gene abolished the infectivity of Cf. The results of this study indica
te that the A protein of Cf is responsible for controlling the host sp
ecificity of Cf.