Identification of the purA gene encoding adenylosuccinate synthetase in Thiobacillus ferrooxidans

Attenuation, persistence, and vaccine potential of an Edwardsiella ictaluri purA mutant

In this study, an adenine-auxotrophic strain of Edwardsiella ictaluri was constructed and its virulence, tissue persistence, and vaccine efficacy were evaluated. A clone containing the purA gene was isolated from an E. ictaluri genomic library, sequenced, and shown to have an overall sequence identity of 79.3% at the nucleotide level and 85.7% at the amino acid level with the Escherichia coli purA gene. The cloned E. ictaluri purA gene was mutated by deleting a 598-bp segment of the gene and inserting the kanamycin resistance gene from Tn903 into the gap. The delta purA::Km(r) gene was subcloned into the suicide plasmid pGP704, and the resulting plasmid was used to deliver the modified gene into a virulent strain of E. ictaluri by conjugation. Homologous recombination replaced the chromosomal purA gene with the mutated gene to create an adenine-auxotrophic strain (LSU-E2). Compared to wild-type E. ictaluri, LSU-E2 was highly attenuated by the injection, immersion, and oral routes of exposure. By the injection route, LSU-E2 had a 50% lethal dose (LD50) that was greater than 5 logs10 higher than the LD50 for wild-type E. ictaluri. In a tissue persistence study, LSU-E2 was able to invade channel catfish by the immersion route and persist in internal organs for at least 48 h. Channel catfish that were vaccinated with a single immersion dose of LSU-E2 had mortality significantly lower (P < 0.01) following a wild-type E. ictaluri challenge than that of nonvaccinated fish.

Molecular genetics of Thiobacillus ferrooxidans

Thiobacillus ferrooxidans is a gram-negative, highly acidophilic (pH 1.5 to 2.0), autotrophic bacterium that obtains its energy through the oxidation of ferrous iron or reduced inorganic sulfur compounds. It is usually dominant in the mixed bacterial populations that are used industrially for the extraction of metals such as copper and uranium from their ores. More recently, these bacterial consortia have been used for the biooxidation of refractory gold-bearing arsenopyrite ores prior to the recovery of gold by cyanidation. The commercial use of T. ferrooxidans has led to an increasing interest in the genetics and molecular biology of the bacterium. Initial investigations were aimed at determining whether the unique physiology and specialized habitat of T. ferrooxidans had been accompanied by a high degree of genetic drift from other gram-negative bacteria. Early genetic studies were comparative in nature and concerned the isolation of genes such as nifHDK, glnA, and recA, which are widespread among bacteria. From a molecular biology viewpoint, T. ferrooxidans appears to be a typical member of the proteobacteria. In most instances, cloned gene promoters and protein products have been functional in Escherichia coli. Although T. ferrooxidans has proved difficult to transform with DNA, research on indigenous plasmids and the isolation of the T. ferrooxidans merA gene have resulted in the development of a low-efficiency electroporation system for one strain of T. ferrooxidans. The most recent studies have focused on the molecular genetics of the pathways associated with nitrogen metabolism, carbon dioxide fixation, and components of the energy-producing mechanisms.