Unraveling the Acidithiobacillus caldus complete genome and its central metabolisms for carbon assimilation

Acidithiobacillus caldus is one of the dominant sulfur-oxidizing bacteria in bioleaching reactors. It plays the essential role in maintaining the high acidity and oxidation of reduced inorganic sulfur compounds during bioleaching process. In this report, the complete genome sequence of A. caldus SM-1 is presented. The genome is composed of one chromosome (2,932,225 bp) and four plasmids (pLAtc1, pLAtc2, pLAtc3, pLAtcm) and it is rich in repetitive sequences (accounting for 11% of the total genome), which are often associated with transposable genetic elements. In particular, twelve copies of ISAtfe and thirty-seven copies of ISAtc1 have been identified, suggesting that they are active transposons in the genome. A. caldus SM-1 encodes all enzymes for the central metabolism and the assimilation of carbon compounds, among which 29 proteins/enzymes were identifiable with proteomic tools. The SM-1 fixes CO2via the classical Calvin–Bassham–Benson (CBB) cycle, and can operate complete Embden-Meyerhof pathway (EMP), pentose phosphate pathway (PPP), and gluconeogenesis. It has an incomplete tricarboxylic acid cycle (TCA). Four putative transporters involved in carbohydrate uptake were identified. Taken together, the results suggested that SM-1 was able to assimilate carbohydrates and this was subsequently confirmed experimentally because addition of 1% glucose or sucrose in basic salt medium significantly increased the growth of SM-1. It was concluded that the complete genome of SM-1 provided fundamental data for further investigation of its physiology and genetics, in addition to the carbon metabolism revealed in this study.