An ice nucleation reporter gene system: Identification of inducible pathogenicity genes in Pseudomonas syringae pv. phaseolicola

We have constructed derivatives of the transposon Tn3 that allow an ice nucleation gene (inaZ) to be used as 'reporter' of the transcriptional activity of genes into which it is inserted. In these derivatives (Tn3-Ice and Tn3-Spice), the lacZYA sequences of transposon Tn3-HoHo1 were replaced with inaZ lacking its native promoter. The ice nucleation activity of virB::inaZ fusions in the correct transcriptional orientation was inducible by acetosyringone, a plant metabolite which activates the vir operon of Agrobacterium tumefaciens Ti plasmids, while fusions in the opposite orientation were unresponsive to the inducer. Tn3-Spice was also used to investigate the expression of a cluster of genes (hrp) which control pathogenicity and hypersensitivity elicited by Pseudomonas syringae pv. phaseolicola. An inducible region was identified which is expressed at low levels in vitro but becomes activated when the bacteria come into contact with the susceptible host, bean. Activation of this region occurred within 2 h post-inoculation and was nearly complete by the time the bacteria began to multiply in the leaf tissue. The inaZ reporter appears to be at least 10⁵-fold more sensitive than lacZ in P.s.phaseolicola. Thus, the inaZ fusion system provides a sensitive, convenient and inexpensive tool for the study of bacterial gene expression, particularly during plant pathogenesis, and should be generally useful as a reporter gene system in Gram-negative bacteria.