FIG. 4.

The hrpX gene of X. oryzae pv. oryzicola controls expression of the hpa1 gene in XOM3 medium. Xanthomonas strains were incubated in XOM3, and the phenotypes of the hrp regulatory gene mutants, hrpX and hrpG, were tested in tobacco leaves (cultivar Nc89) (A) and in rice (IR24) (C), respectively. The induction of hpa1 expression in XOM3 medium was identified by RT-PCR (B), which was performed using total RNA as the template. The hrpX mutant RCX (1), the hrpG mutant RCG (2), and the wild-type strain RS105 (3) were grown at 28°C in XOM3 (pH 6.5). Total RNA was extracted from each bacterium after incubation for 16 h. Specific primers that amplified a 414-bp DNA fragment corresponding to the hpa1 gene were used for RT-PCR. PCR products (B, top) and rRNA (B, bottom) were separated by agarose gel electrophoresis and stained with ethidium bromide. A positive PCR control using RS105 gDNA as the template (4) and a negative PCR control with no RT (5) are also indicated.