phiL7 is a lytic phage infecting the gram-negative Xanthomonas campestis pv. campestris, a plant pathogen. To study phage-host interaction, a phiL7-resistant mutant was isolated from strain Xc17 by mini-Tn5 transposition and designated CH7LR. CH7LR could not plate phiL7 in double-layered assay and formed turbid clearing zones when the cell lawn was dropped with a high titer of phiL7. Sequence analysis showed that the mutated gene is xanA coding for phosphoglucomutase/phosphomannomutase, required for the synthesis of lipopolysaccharide and exopolysaccharide (xanthan). The involvement of xanA was confirmed by isolating another mutant with interrupted xanA and complementing with the cloned wild-type gene. Nonmucoid mutants are still sensitive to phiL7 indicating that xanthan is not involved in phiL7 adsorption. Since the mutants still exhibited low efficiencies of phage adsorption, we predict, by analogy with the cases in other bacteriophages of gram-negative bacteria, that other outer membrane components such as a protein are required for the formation of a complex receptor. (C) 2002 Elsevier Science (USA).