Book excerpt: A pathogenicity locus of Pseudomonas syringae pv. syringae identified by Tn5 mutagenesis was investigated. The mutant strain PS9024 is attenuated for disease expression in its host, Phaseolus vulgaris, but produces the hypersensitive reaction (HR) in the nonhost, tobacco (Nicotina tabacum). A cosmid clone carrying 16 kilobases (kb) of contiguous genomic DNA partially complements this mutant. Altered growth of the mutant in planta was also partially restored. Marker exchange mutagenesis with Tn3- HoHo1 at two other sites within this locus results in mutants with attenuated and severely reduced pathogenicity. The locus is complex and contains repetitive DNA sequences. Northern analysis reveals that this locus is expressed in planta, but is not expressed in a rich growth medium, and the transcript is larger than 10 kb, suggesting that the locus is transcribed as a polycistronic mRNA. Comparison of total cellular protein profiles of R32 and PS9024 using SDS-PAGE analysis further reveals that at least nine protein bands ranging from approximately 100 kD or above in size are present in the wild type strain R32, but absent from the mutant. Additionally, a protein of approximately 45 kD is absent from the mutant. The site of Tn5 insertion has been partially sequenced. The initial search of the data banks suggested a gene or genes related to the ornithine biosynthetic pathway map to this locus. Further study strongly suggest a gene that encodes a membranceassociated protein and under the control of a promoter identical to appA gene promoter maps at this site and it is involved in the process of pathogenesis.