Binds mRNA, facilitating recognition of most mRNAs by the 30S ribosomal subunit during translation initiation (By similarity). Plays a role in trans-translation; binds tmRNA (the product of the ssrA gene) (PubMed:25430994). Binds very poorly to pyrazinoic acid (POA), the active form of the prodrug pyrazinamide (PZA); POA does not disrupt trans-translation in this organism (PubMed:21835980). M.smegmatis is resistant to the antibiotic PZA (PubMed:25430994). In trans-translation Ala-aminoacylated transfer-messenger RNA (tmRNA, product of the ssrA gene; the 2 termini fold to resemble tRNA(Ala) while it encodes a short internal open reading frame (the tag peptide)) acts like a tRNA, entering the A-site of the ribosome and displacing the stalled mRNA (which is subsequently degraded). The ribosome then switches to translate the ORF on the tmRNA, the nascent peptide is terminated with the 'tag peptide' encoded by the tmRNA and thus targeted for degradation. Binds uncharacterized protein MSMEG_2731/MSMEI_2664. Full-length S1 protein and the C-terminal domain (residues 285-479) bind tmRNA, the interaction is not disrupted by POA. Unlike M.tuberculosis (strain H37Rv) the S1 motif 4 does not bind POA, which may explain why M.smegmatis is resistant to the antibiotic PZA, the precursor to POA. Belongs to the bacterial ribosomal protein bS1 family.