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

Abstract

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 of X. oryzae pv. oryzae restored Eps production and virulence to several spv (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, into gumM, the last gene of the gum gene cluster. The results of Southern analysis indicate the presence of multiple copies of both IS elements in the genome of X. oryzae pv. oryzae. These results demonstrate the role of IS elements in stationary-phase variation in X. oryzae pv. oryzae.In their natural environment, bacteria often encounter nutritionally limited conditions which resemble the stationaryphase conditions of laboratory cultures. Gram-positive bacteria respond to the adverse conditions by forming resistant spores. Gram-negative bacteria exhibit reduced growth, and several genes that confer resistance to stress during starvation are transcriptionally induced in the stationary phase. Much of the transcriptional regulation in the stationary phase is mediated by rpoS, a gene that encodes the stationary-phase sigma factor ( S ) of RNA polymerase (14, 21). Several Escherichia coli mutants that have a competitive advantage in stationaryphase cultures have been shown to carry mutations in the rpoS gene (38).Spontaneous and reversible phenotypic variations mediated by various DNA rearrangements, such as insertions, DNA duplications, inversions, deletions, and frameshift mutations, are strategies adopted by bacteria to cope with adverse environmental conditions (7). A few examples are intragenic duplications in a regulatory gene in the mushroom pathogen Pseudomonas tolaasii (11), large chromosomal duplications that promote growth during carbon starvation in Salmonella enterica serovar Typhimurium (34), insertions in Shigella flexneri (24), frameshift mutations in Bordetella pertussis (35), and flagellar-phase variation mediated by DNA inversion in serovar Typhimurium (39). In the well-studied plant-pathogenic bacterium Ralstonia solanacearum, phenotypic changes were shown to be due to either mutations in the phcA regulatory gene (2), overproduction of a negative regulatory element (16), or excision of an episome from the bacterial chromosome (26).Xanthomonas oryzae pv. oryzae causes bacterial leaf blight, a serious disease of rice. Like other xanthomonads, this bacterium produces copious amounts of an extracellular polysaccharide (Eps). The Eps produced by a related bacterium, Xanthomonas campestris pv. campestris, is the industrially important xanthan gum, whi...

A LuxR Homolog Controls Production of Symbiotically Active Extracellular Polysaccharide II by Sinorhizobium meliloti

“…Some examples of this include Neisseria meningitidis switching from a capsule-producing form to a capsule-deficient form (38), IS-mediated reversible production of EPS in Pseudoalteromonas atlantica (5), and phase variation of Xanthomonas oryzae pv. oryzae with regard to xanthan gum production (60). It is possible that, in the field, S. meliloti uses the ISRm2011-1 insertion in expR as a genetically plastic switch to control EPS II production.…”

Identification of differentially expressed genes of Xanthomonas axonopodis pv. citri by representational difference analysis of cDNA

“…oryzae revealed that spontaneous mutants deficient for virulence and extracellular polysaccharides accumulate in the stationary phase. Results of these studies showed that these mutations occur due to insertion sequences (IS) (Rajeshwari and Sonti, 2000). After 12 h growth in NYG, X. axonopodis pv.…”

Molecular determinants of disease and resistance in interactions of Xanthomonas oryzae pv. oryzae and rice

“…oryzae is that it produces copious amounts of EPS when cultured in medium supplemented with sucrose. Spontaneous loss of virulence associated with reduction of EPS production has been observed in long-term storage and culture [11]. Therefore, EPS is regarded as a virulence determinant.…”

“…Mutants of X. oryzae pv. oryzae gumM recovered from stationary-phase culture are also deficient for virulence and EPS production [11]. Recently by Tn5 mutagenesis, a novel genomic locus which is not located in the gum cluster has been identified to be required for EPS and virulence in X. oryzae pv.…”

“…oryzae pv. oryzae interaction, with the cloning of the two rice resistance (R) genes, Xa1 and Xa21 [4,5], the bacterial avirulence (avr) genes, avrXa5, avrXa7 and avrXa10 [6], the hypersensitive response and pathogenicity (hrp) genes [7] and many other virulencerelated genes [8][9][10][11][12][13][14]. Studies on the pathogen population structure using molecular genotyping methods have provided information on pathogen evolution.…”