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Motility is an essential characteristic for mesophilic Aeromonas strains. We identified a new polar flagellum region (region 6) in the A. hydrophila AH-3 (serotype 034) chromosome that contained two additional polar stator genes, named pom[A.sub.2] and pom[B.sub.2]. A. hydrophila Pom[A.sub.2] and Pom[B.sub.2] are highly homologous to other sodium-conducting polar flagellum stator motors as well as to the previously described A. hydrophila AH-3 PomA and PomB. pomAB and pom[A.sub.2][B.sub.2] were present in all the mesophilic Aeromonas strains tested and were independent of the strains' ability to produce lateral flagella. Unlike MotX, which is a stator protein that is essential for polar flagellum rotation, here we demonstrate that PomAB and Pom[A.sub.2][B.sub.2] are redundant sets of proteins, as neither set on its own is essential for polar flagellum motility in either aqueous or high-viscosity environments. Both PomAB and Pom[A.sub.2][B.sub.2] are sodium.coupled stator complexes, although Pom[A.sub.2][B.sub.2] is more sensitive to low concentrations of sodium than PomAB. Furthermore, the level of transcription in aqueous and high-viscosity environments of pom[A.sub.2][B.sub.2] is reduced compared to that of pomAB. The A. hydrophila AH-3 polar flagellum is the first case described in which two redundant sodium-driven stator motor proteins (PomAB and Pom[A.sub.2][B.sub.2]) are found.

Comparison between the lipopolysaccharide (LPS) core structures of Aeromonas salmonicida subsp, salmonicida A450 and Aeromonas hydrophila AH-3 shows great similarity in the inner LPS core and part of the outer LPS core but some differences in the distal part of the outer LPS core (residues LD-Hep, D-Gal, and D-GalNAc). The three genomic regions encoding LPS core biosynthetic genes in A. salmonicida A450, of which regions 2 and 3 have genes identical to those of A. hydrophila AH-3, were fully sequenced. A. salmonicida A450 region 1 showed seven genes: three identical to those of A. hydrophila AH-3, three similar but not identical to those of A. hydrophila AH-3, and one without any homology to any well-characterized gene. A. salmonicida A450 mutants with alterations in the genes that were not identical to those of A. hydrophila AH-3 were constructed, and their LPS core structures were fully elucidated. At the same time, all the A. salmonicida A450 genes identical to those of A. hydrophila AH-3 were used to complement the previously obtained A. hydrophila AH-3 mutants for each of these genes. Combining the gene sequence and complementation test data with the structural data and phenotypic characterization of the mutant LPSs enabled a presumptive assignment of all LPS core biosynthesis gene functions in A. salmonicida A450. Furthermore, hybridization studies with internal probes for the A. salmonicida-specific genes using different A. salmonicida strains (strains of different subspecies or atypical strains) showed a unique or prevalent LPS core type, which is the one fully characterized for A. salmonicida A450.

The Aeromonas hydrophila [wb.sub.*O34] gene cluster of strain AH-3 (serotype O34) was cloned and sequenced. This cluster contains genes necessary for the production of O34-antigen lipopolysaccharide (LPS) in A. hydrophila. We determined, using either mutation or sequence homology, roles for the majority of genes in the cluster by using the chemical O34-antigen LPS structure obtained for strain AH-3. The O34-antigen LPS export system has been shown to be a Wzy-dependent pathway typical of heteropolysaccharide pathways. Furthermore, the production of A. hydrophila O34-antigen LPS in Escherichia coli K-12 strains is dependent on incorporation of the Gne enzyme (UDP-N-acetylgalactosamine 4-epimerase) necessary for the formation of UDP-galactosamine in these strains. By using rapid amplification of cDNA ends we were able to identify a transcription start site upstream of the terminal wzz gene, which showed differential transcription depending on the growth temperature of the strain. The Wzz protein is able to regulate the O34-antigen LPS chain length. The differential expression of this protein at different temperatures, which was substantially greater at 20[degrees]C than at 37[degrees]C, explains the previously observed differential production of O34-antigen LPS and its correlation with the virulence of A. hydrophila serotype O34 strains.