Stationary-Phase Variation Due to Transposition of Novel Insertion Elements in Xanthomonas oryzae pv. oryzae

Xanthomonas oryzae pv. oryzae causes bacterial leaf blight, a serious disease of rice. Spontaneous mutants which are deficient for virulence and extracellular polysaccharide (Eps) production accumulate in large numbers in stationary-phase cultures of this bacterium, a phenomenon which we have called stationary-phase variation. A clone (pSD1) carrying the Eps biosynthetic gene (gum ) cluster ofX. oryzae pv. oryzae restored Eps production and virulence to severalspv (for stationary-phase variation) mutants. Data from localized recombination analysis, Southern hybridization, PCR amplification, and sequence analysis showed that the mutations are due to insertion of either one of two novel endogenous insertion sequence (IS) elements, namely, ISXo1 and ISXo2 , intogumM , the last gene of thegum gene cluster. The results of Southern analysis indicate the presence of multiple copies of both IS elements in the genome ofX. oryzae pv. oryzae. These results demonstrate the role of IS elements in stationary-phase variation inX. oryzae pv. oryzae.