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Background   Acidithiobacillus ferrooxidans  is chemolithoautotrophic γ-proteobacterium that thrives at extremely low pH (pH 1-2). Although a substantial amount of information is available regarding CO 2  uptake and fixation in a variety of facultative autotrophs, less is known about the processes in obligate autotrophs, especially those living in extremely acidic conditions, prompting the present study.    Results  Four gene clusters (termed  cbb1-4 ) in the  A. ferrooxidans  genome are predicted to encode enzymes and structural proteins involved in carbon assimilation via the Calvin-Benson-Bassham (CBB) cycle including form I of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO, EC 4.1.1.39) and the CO 2 -concentrating carboxysomes. RT-PCR experiments demonstrated that each gene cluster is a single transcriptional unit and thus is an operon. Operon  cbb1  is divergently transcribed from a gene,  cbbR , encoding the LysR-type transcriptional regulator CbbR that has been shown in many organisms to regulate the expression of RubisCO genes. Sigma 70 -like -10 and -35 promoter boxes and potential CbbR-binding sites (T-N 11 -A/TNA-N 7 TNA) were predicted in the upstream regions of the four operons. Electrophoretic mobility shift assays (EMSAs) confirmed that purified CbbR is able to bind to the upstream regions of the  cbb1 ,  cbb2  and  cbb3  operons, demonstrating that the predicted CbbR-binding sites are functional  in vitro . However, CbbR failed to bind the upstream region of the  cbb4  operon that contains  cbbP , encoding phosphoribulokinase (EC 2.7.1.19). Thus, other factors not present in the assay may be required for binding or the region lacks a functional CbbR-binding site. The  cbb3  operon contains genes predicted to encode anthranilate synthase components I and II, catalyzing the formation of anthranilate and pyruvate from chorismate. This suggests a novel regulatory connection between CO 2  fixation and tryptophan biosynthesis. The presence of a form II RubisCO could promote the ability of  A. ferrooxidans  to fix CO 2  at different concentrations of CO 2 .    Conclusions   A. ferrooxidans  has features of  cbb  gene organization for CO 2 -assimilating functions that are characteristic of obligate chemolithoautotrophs and distinguish this group from facultative autotrophs. The most conspicuous difference is a separate operon for the  cbbP  gene. It is hypothesized that this organization may provide greater flexibility in the regulation of expression of genes involved in inorganic carbon assimilation.

bmc microbiology

Genes and pathways for CO2 fixation in the obligate, chemolithoautotrophic acidophile, Acidithiobacillus ferrooxidans, Carbon fixation in A. ferrooxidans