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Involvement of Pseudomonas aeruginosa Rhodanese in Protection from Cyanide Toxicity

Rita Cipollone,¹ Emanuela Frangipani,^1, Federica Tiburzi,^1,2 Francesco Imperi,^1,2 Paolo Ascenzi,^1,2 and Paolo Visca^1,2*

Dipartimento di Biologia, Università Roma Tre, Viale G. Marconi 446, 00146 Rome, Italy,¹ Istituto Nazionale per le Malattie Infettive I.R.C.C.S. Lazzaro Spallanzani, Via Portuense 292, 00149 Rome, Italy²

Received 12 September 2006/ Accepted 1 November 2006

Cyanide is a serious environmental pollutant and a biocontrol metabolite in plant growth-promoting Pseudomonas species. Here we report on the presence of multiple sulfurtransferases in the cyanogenic bacterium Pseudomonas aeruginosa PAO1 and investigate in detail RhdA, a thiosulfate:cyanide sulfurtransferase (rhodanese) which converts cyanide to less toxic thiocyanate. RhdA is a cytoplasmic enzyme acting as the principal rhodanese in P. aeruginosa. The rhdA gene forms a transcriptional unit with the PA4955 and psd genes and is controlled by two promoters located upstream of PA4955 and rhdA. Both promoters direct constitutive RhdA expression and show similar patterns of activity, involving moderate down-regulation at the stationary phase or in the presence of exogenous cyanide. We previously observed that RhdA overproduction protects Escherichia coli against cyanide toxicity, and here we show that physiological RhdA levels contribute to P. aeruginosa survival under cyanogenic conditions. The growth of a rhdA mutant is impaired under cyanogenic conditions and fully restored upon complementation with rhdA. Wild-type P. aeruginosa outcompetes the rhdA mutant in cyanogenic coculture assays. Hence, RhdA could be regarded as an effector of P. aeruginosa intrinsic resistance to cyanide, insofar as it provides the bacterium with a defense mechanism against endogenous cyanide toxicity, in addition to cyanide-resistant respiration.

Published ahead of print on 10 November 2006.

Present address: Departement de Microbiologie Fondamentale, Université de Lausanne, CH-1015 Lausanne, Switzerland.