Binds mRNA, facilitating recognition of most mRNAs by the 30S ribosomal subunit during translation initiation (By similarity). Probably plays a role in trans-translation; binds tmRNA (the product of the ssrA gene) (PubMed:21835980). 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 (By similarity). Binds pyrazinoic acid (POA), the active form of the prodrug pyrazinamide (PZA); POA disrupts tmRNA binding (at 50 ug/ml in vitro) and also trans-translation (at 25 ug/ml in vitro using purified H37Ra ribosomes). Full-length S1 protein and the C-terminal domain (residues 285-481) bind tmRNA; the interaction is disrupted in a concentration-dependent manner by POA. 2 POA molecules bind to the C-terminal domain (residues 285-481) and S1 motif 4 (residues 285-368). Mutations in this gene that alter POA binding may cause antibiotic resistance. Belongs to the bacterial ribosomal protein bS1 family.