Trans-translation ensures timely initiation of DNA replication and DnaA synthesis in Escherichia coli

The trans-translation pathway, mediated by the transfer messenger RNA (tmRNA; encoded by the ssrA gene) and the SmpB protein (tmRNA-binding protein expressed in Salmonella enterica), which is conserved in bacteria, is required for various cellular processes. A previous study has shown that trans-translation is required to ensure timely (non-delayed) dnaA transcription and consequent initiation of DNA replication in Caulobacter crescentus. In this study, we observed that initiation of chromosome replication was delayed in Escherichia coli lacking the smpB and/or ssrA genes (DssrA, DsmpB, or DsmpBDssrA mutants). We observed that the growth rate of the mutant cells was much slower than that of its wild-type counterpart. However, the delayed initiation of replication and slower growth in the DssrA or DsmpB mutants were reversed by ectopic expression of tmRNA or SmpB. A synchronized DsmpBDssrA cell culture containing the dnaC2 mutant allele showed delayed protein (total and DnaA) accumulation per cell; DnaA accumulation was also delayed in the DsmpB. These results indicated that absence of trans-translation leads to a delay in initiation of DNA replication, synthesis of total protein (including DnaA), and a decrease in E. coli growth rate. In summary, we propose that the trans-translation pathway is required to ensure timely initiation of replication, protein synthesis, and subsequent cell cycle progression.