The copper-inducible cin operon encodes an unusual methionine-rich azurin-like protein and a pre-Q₀ reductase in Pseudomonas putida KT2440

The genome sequences of several pseudomonads have revealed a gene cluster containing genes for a two-component heavy metal histidine sensor kinase and response regulator upstream of cinA and cinQ, which we show herein to encode a copper-containing azurin-like protein and a pre-Q₀ reductase, respectively. In the presence of copper, Pseudomonas putida KT2440 produces the CinA and CinQ proteins from a bicistronic mRNA. UV-visible spectra of CinA show features at 439, 581, and 719 nm, which is typical of the plastocyanin family of proteins. The redox potential of the protein was shown to be 456 ± 4 mV by voltametric titrations. Surprisingly, CinQ is a pyridine nucleotide-dependent nitrile oxidoreductase that catalyzes the conversion of pre-Q₀ to pre-Q₁ in the nucleoside queuosine biosynthetic pathway. Gene disruptions of cinA and cinQ did not lead to a significant increase in the copper sensitivity of P. putida KT2440 under the conditions tested. Possible roles of CinA and CinQ to help pseudomonads adapt and survive under prolonged copper stress are discussed.