Initial phases for the YahD crystal data were obtained by molecular replacement using molrep of the ccp4 program suite (Collaborative Computational Project, Number 4, 1994; Vagin & Teplyakov, 2010). The model obtained was subjected to rigid-body refinement, followed by iterative cycles of restrained-maximum likelihood refinement, including Linsitinib isotropic temperature factor

adjustment with refmac (Murshudov et al., 1997) and by manual rebuilding using coot (Emsley & Cowtan, 2004). During this process, 5% randomly selected reflections have been used to calculate Rfree to monitor bias during model building and refinement. Water molecules were added using coot, and the validation of the model was carried out using molprobity. The atomic coordinates and structure factors have been deposited in the Protein Data Bank under accession 3OG9. We previously identified yahD as a copper-induced gene of L. lactis IL1403 and, PD-166866 here, aimed to characterize the corresponding gene product. By visual inspection and bioinformatics analysis (Ermolaeva et al., 2001), the gene encoding YahD is predicted to be part of an operon consisting of yahC, yahD,

yaiA and yaiB (Fig. 1). The operon is preceded by a cop box of consensus TACANNTGTA, which has been shown previously to interact with the copper-responsive repressor, CopR, of L. lactis. The operon terminates in a hairpin loop (theoretical stability however −16.8 kcal), which presumably acts as a ρ-independent transcriptional

terminator. The presence of these transcriptional control elements, together with the dense spacing of the four genes, further supports the operon structure. The first gene of the operon, yahC, encodes a hypothetical protein of 65 amino acids (accession NP_835288), followed by yahD (accession NP_266234), predicted to encode a serine hydrolase of 206 amino acids. The final two genes of the operon are yaiA (accession NP_266233), encoding a predicted protein of 389 amino acids with sequence similarity to glyoxylases I (lactoylglutathione lyases), and yaiB (accession NP_266234), encoding a hypothetical protein of 196 amino acids. All proteins of the operon have calculated pI values in the range of 4.5–5. Because bacterial genes are usually grouped in operons based on metabolic relationships, we also studied the operon context of yahD-like genes in related organisms. The L. lactis operon and the operons of five other well-studied Firmicutes, namely Enterococcus faecalis V583, Staphylococcus aureus N315, B. cereus E33L, Bacillus subtilis 168 and Lactobacillus casei BL23, were compared (Fig. 1). All six operons feature the expected −10 and −35 sequence elements and are also terminated by stem–loop structures with stabilities of −10.9 to −27.7 kcal mol−1. Interestingly, the yahC gene is unique to L.