Use of plasmid pULB113 (RP4::Mini-Mu) to construct a genomic map ofAeromonas hydrophila

The gene encoding a periplasmic deoxyribonuclease from Aeromonas hydrophila

A gene encoding a deoxyribonuclease, dnsH, was cloned from Aeromonas hydrophila JMP636. The predicted mature protein was very similar to the previously described extracellular Dns from this organism and an N-terminal region corresponding to a large putative signal sequence was predicted for the JMP636 protein. Inactivation of dnsII demonstrated that the DnsH protein was not present extracellularly in this strain. As DnsH degraded plasmid DNA and was believed to have a periplasmic location, a dnsH mutant was constructed to determine whether electroporation of A. hydrophila with plasmid DNA could be achieved. No transformants were detected. From SDS-PAGE studies, at least two additional DNases remain to be characterised from A. hydrophila JMP636.

Construction of a physical and preliminary genetic map of Aeromonas hydrophila JMP636

A physical and preliminary genetic map of the Aeromonas hydrophila JMP636 chromosome has been constructed. The topology of the genome was predicted to be circular as chromosomal DNA did not migrate from the origin during PFGE unless linearized by S1 nuclease. Cleavage of the chromosome with PacI and PmeI produced 23 and 14 fragments, respectively, and enabled calculation of the genome size at 4.5 Mb. Digestion of the chromosome with I-CeuI produced 10 fragments, indicating that 10 rrl (23S) genes were likely to be present. Hybridizations between DNA fragments generated with PacI, PmeI and I-CeuI were used to initially determine the relationship between these segments. To accurately map genes previously characterized from JMP636, the suicide vector pJP5603 was modified to introduce restriction sites for PacI and PmeI, producing pJP9540. Following cloning of genes into this vector and recombinational insertion into the JMP636 chromosome, PacI and PmeI cleavage determined the location of genes within macrorestriction fragments with the additional bands produced forming hybridization probes. From the data generated, it was possible to form a physical map comprising all the fragments produced by PacI and PmeI, and assign the contig of I-CeuI fragments on this map. The preliminary genetic map defines the location of six loci for degradative enzymes previously characterized from JMP636, while the locations of the 10 sets of ribosomal genes were assigned with less accuracy from hybridization data.

A lipase of Aeromonas hydrophila showing nonhemolytic phospholipase C activity

Extracellular lipase activity detected on tributyrin agar has been identified in a cosmid clone, JM3084, constructed from the chromosome of Aeromonas hydrophila and vector pHC79. This lipase, named apl-1, also exhibits nonhemolytic phospholipase C activity on lecithin and p-nitrophenylphosphorylcholine. Subcloning of the cosmid JMP3084 with partial Sau3a1 digestion localized the lipase gene to a 3.4-kb DNA fragment. Southern blot analysis shows the gene apl-1 to exist in single copy on the A. hydrophila chromosome. Expression of apl-1 in the pT7 system identified a single protein of molecular weight 70 kDa. Nucleotide sequencing of apl-1 has identified an open reading frame of 2055 bases predicting a protein of 73 kDa. The presence of an amino terminal signal sequence of 18 amino acids accounts for this molecular weight disparity. Further analysis of the lipase amino acid sequence revealed the presence of a classical serine active lipase site (Gly-X-Ser-X-Gly) located between residues 561 and 570. The A. hydrophila chromosomal copy of apl-1 has been inactivated by use of the mutagenesis vector pJP5603, resulting in the complete removal of phospholipase C activity and lowered levels of lipase activity detected on tributyrin agar.

Molecular cloning, characterization, and nucleotide sequence of an extracellular amylase gene from Aeromonas hydrophila

The structural gene for excreted amylase from Aeromonas hydrophila JMP636 has been cloned within a 2.1-kilobase SmaI fragment of DNA. The amylase gene is transcribed from its own promoter in Escherichia coli, producing a gene product of Mr 49,000. The amylase gene product is secreted to the periplasm of E. coli; however, it is not excreted. Nucleotide sequencing revealed an open reading frame of 1,392 base pairs corresponding to a protein of 464 amino acid residues. A potential signal peptide of 21 amino acid residues is present at the NH2 terminal of the predicted protein. Three regions of homology with other procaryotic and eucaryotic alpha-amylases were detected within the predicted amino acid sequence.