Department of Plant Pathology, Cornell University, Ithaca, NY 14853-4203, U.S.A.
The pelS gene from Pseudomonas syringae pv. lachrymans 859 was cloned by heterologous expression in nonpectolytic P. syringae pv. syringae BUVS1, using genomic DNA libraries constructed with two novel broad-host-range cosmid vectors, pCPP34 and pCPP47. Screening of P. syringae pv. syringae transconjugants for the ability to pit pectate media at pH 6.0 and 8.5 yielded several overlapping clones of the same DNA region. Ultrathin-layer isoelectric focusing gels, activity-stained with diagnostically buffered substrate overlays, revealed that this region encoded a single pectate lyase (PelS) with a pI of 9.4. pelS was subcloned from cosmid pCPP5020 and sequenced, revealing it to encode a member of the Erwinia chrysanthemi PelADE family, with highest similarity to Pseudomonas viridiflava PelV. A pelS probe hybridized at high stringency in DNA gel blots with total DNA from P. syringae pv. lachrymans strains 859 and Pla5, P. syringae pv. tabaci, P. syringae pv. phaseolicola, P. syringae pv. glycinea, P. fluorescens (marginalis), P. viridiflava, and Xanthomonas campestris pv. campestris, but not with P. syringae pv. pisi, P. syringae pv. syringae, P. syringae pv. tomato, P. syringae pv. papulans, E. chrysanthemi, or Ralstonia (Pseudomonas or Burkholderia) solanacearum. The PelS sequence revealed an N-terminal signal peptide, whose processing in Escherichia coli was confirmed by protein sequence analysis. PelS was similar to E. chrysanthemi PelE in its substrate preference and ability to reduce the viscosity of pectate and to macerate potato tuber tissue. A pelS∷ΩKmr mutation was marker-exchanged into P. syringae pv. lachrymans Pla5. pelS was also subcloned into the broad-host-range expression vector pML122 under control of the vector nptII promoter, and then transformed into P. syringae pv. lachrymans Pla5 to produce a strain overproducing PelS. Necrotic lesions developed in cotyledons following inoculation with all of the P. syringae pv. lachrymans Pla5 derivatives, regardless of their Pel phenotype. However, only cotyledons infected with pelS+ strains showed evidence of maceration and yielded Pel activity upon extraction. In contrast, pelS+ P. syringae pv. syringae BUVS1(pCPP5020) produced no symptoms in cucumber cotyledons. Thus, PelS in P. syringae pv. lachrymans appears to alter the final symptoms in infected cucumber cotyledons without contributing to pathogenicity or altering host range.