Molecular techniques and sequencing Plasmids pILL788, pILL791, pILL792, pILL793, pILL794, pILL795, pILL2328 correspond to H. pylori ssrA WT , ssrA DD , ssrA resume , ssrA wobble , ssrA smpB , ssrA STOP genes cloned into the E. coli/H. pylori shuttle vector pILL2150 [24], respectively. SsrA mutagenesis has been described in [10]. The H. pylori ssrA gene check details amplified by PCR with primers H367 (5′-CGGGATCCCTCACCTGTTCTTTCTGA-3′) and H368 (5′-GGGGTACCCGGATCCTT AATCGAATAAAAATCAGG-3′) was cloned into the pEXT21 low copy number vector (1-3 copies per cell) [25] using BamHI/KpnI

restriction sites (Table 1). The resulting plasmid was designated pILL2318. The E. coli ssrA gene amplified by PCR with primers H365 5′-CTATCCCGGCGC TGGGTAACATCGGG-3, and H366 5′-GCTTTTCGTTGGGCCTATCAATGGGCC-3′ was cloned into pILL2150, to generate pILL2334. The H. pylori smpB

gene amplified by PCR with primers H225 (5′-GGACTAGTAGGAAGAGAATAATGAAACTCATTGCCAG CAAC-3′) and H236 (5′-CGGGGTACCTTATCCTTTAAAGTGGTGTTTTAAATCAGC-3′), was cloned into pILL2150 [24] using SpeI/KpnI restriction sites to generate pILL786. Test of λimmP22 propagation in E. coli The efficiency of plating (EOP) strains was determined by plating tenfold serial dilution of phage λimm P22 on top agar mixed with 100 μl E. coli overnight liquid culture in LB with 0.4% maltose and 10 mM MgSO4. The number of CFU·ml-1 was calculated for each E. coli strain. The EOP is the ratio between the titer of phage on a bacterial lawn of the indicated strain (Table PD98059 in vivo 3) and that of the wild type strain. Western blot Western blot to detect SmpB proteins was performed with E. coli whole cell sonicates prepared as in [26]. Protein Orotidine 5′-phosphate decarboxylase concentrations were measured with Bradford assay (Bio-Rad). Twenty μg of crude extracts were separated by 15% SDS-PAGE and blotted on a polyvinylidene difluororide membrane (PVDF, Millipore). Hp-SmpB and Ec-SpmB were detected

with rabbit polyclonal antibody raised against Ec-SmpB (a generous gift of B. Felden). Binding of the IgG anti-rabbit coupled peroxydase antibody (Amersham) was revealed with the ECL Plus reagent (Pierce). RNA extraction, riboprobe synthesis and northern blot RNAs were extracted using the phenol-chloroform method as described in [27]. An E. coli 5S rRNA riboprobe was synthesized using both primers H357 (5-GCCTGGCGGCAGTAGCG CG GTGG-3′) and H358 (5′-CTAATACGACTCACTATAGGGAGAGCCTGGCAGTTCCC TACTCTCGC-3′). Riboprobes synthesis for H. pylori SsrA was as in [10]. The ladder used corresponds to pBR322 vector digested by MspI and labeled at the 5′end with γ 32P ATP. Intensities of the bands were determined with Quantity One Software (Bio-Rad). The northern blot procedure was as described in [10]. Acknowledgements The authors thank A. Labigne for her support. We also want to thank B. Felden for the gift of anti-EcSmpB antibodies and for constructive comments. We are grateful to J. Collier and P. Bouloc for the gift of E. coli strains MG1655ΔssrA and ΔsmpB and to H. Neil, K. Zemam and C.