The small heat shock proteins from Acidithiobacillus ferrooxidans: gene expression, phylogenetic analysis, and structural modeling

Background: Acidithiobacillus ferrooxidans is an acidophilic, chemolithoautotrophic bacterium that has been successfully used in metal bioleaching. In this study, an analysis of the A. ferrooxidans ATCC 23270 genome revealed the presence of three sHSP genes, Afe_1009, Afe_1437 and Afe_2172, that encode proteins from the HSP20 family, a class of intracellular multimers that is especially important in extremophile microorganisms. Results: The expression of the sHSP genes was investigated in A. ferrooxidans cells submitted to a heat shock at 40 degrees C for 15, 30 and 60 minutes. After 60 minutes, the gene on locus Afe_1437 was about 20-fold more highly expressed than the gene on locus Afe_2172. Bioinformatic and phylogenetic analyses showed that the sHSPs from A. ferrooxidans are possible non-paralogous proteins, and are regulated by the sigma(32) factor, a common transcription factor of heat shock proteins. Structural studies using homology molecular modeling indicated that the proteins encoded by Afe_1009 and Afe_1437 have a conserved alpha-crystallin domain and share similar structural features with the sHSP from Methanococcus jannaschii, suggesting that their biological assembly involves 24 molecules and resembles a hollow spherical shell. Conclusion: We conclude that the sHSPs encoded by the Afe_1437 and Afe_1009 genes are more likely to act as molecular chaperones in the A. ferrooxidans heat shock response. In addition, the three sHSPs from A. ferrooxidans are not recent paralogs, and the Afe_1437 and Afe_1009 genes could be inherited horizontally by A. ferrooxidans.