Construction of arsB and tetH mutants of the sulfur-oxidizing bacterium Acidithiobacillus caldus by marker exchange.

Acidithiobacillus caldus is a moderately thermophilic, acidophilic bacterium that has been reported to be the dominant sulfur oxidizer in stirred-tank processes used to treat gold-bearing arsenopyrite ores. It is also widely distributed in heap reactors used for the extraction of metals from ores. Not only are these bacteria commercially important, they have an interesting physiology, the study of which has been restricted by the nonavailability of defined mutants. A recently reported conjugation system based on the broad-host-range IncW plasmids pSa and R388 was used to transfer mobilizable narrow-host-range suicide plasmid vectors containing inactivated and partially deleted chromosomal genes from Escherichia coli to A. caldus. Through the dual use of a selectable kanamycin resistance gene and a hybridization probe made from a deleted portion of the target chromosomal gene, single- and double-recombinant mutants of A. caldus were isolated. The functionality of the gene inactivation system was shown by the construction of A. caldus arsB and tetH mutants, and the effects of these mutations on cell growth in the presence of arsenic and by means of tetrathionate oxidation were demonstrated.