Effects of ribosomal proteins S1, S2 and the DeaD/CsdA DEAD‐box helicase on translation of leaderless and canonical mRNAs in <i>Escherichia coli</i>

<jats:title>Summary</jats:title><jats:p>Leaderless mRNAs beginning with the AUG initiating codon occur in all kingdoms of life. It has been previously reported that translation of the leaderless λcI mRNA is stimulated in an <jats:italic>Escherichia coli rpsB</jats:italic> mutant deficient in ribosomal protein S2. Here, we have studied this phenomenon at the molecular level by making use of an <jats:italic>E. coli rpsB<jats:sup>ts</jats:sup></jats:italic> mutant. The analysis of the ribosomes isolated under the non‐permissive conditions revealed that in addition to ribosomal protein S2, ribosomal protein S1 was absent, demonstrating that S2 is essential for binding of S1 to the 30S ribosomal subunit. <jats:italic>In vitro</jats:italic> translation assays and the selective translation of a leaderless mRNA <jats:italic>in vivo</jats:italic> at the non‐permissive temperature corroborate and extend previous <jats:italic>in vitro</jats:italic> ribosome binding studies in that S1 is indeed dispensable for translation of leaderless mRNAs. The <jats:italic>deaD/csdA</jats:italic> gene, encoding the ‘DeaD/CsdA’ DEAD‐box helicase, has been isolated as a multicopy suppressor of <jats:italic>rpsB<jats:sup>ts</jats:sup></jats:italic> mutations. Here, we show that expression of a plasmid borne <jats:italic>deaD/csdA</jats:italic> gene restores both S1 and S2 on the ribosome at the non‐permissive temperature in the <jats:italic>rpsB<jats:sup>ts</jats:sup></jats:italic> strain, which in turn leads to suppression of the translational defect affecting canonical mRNAs. These data are discussed in terms of a model, wherein DeaD/CsdA is involved in ribosome biogenesis rather than acting directly on mRNA.</jats:p>