Binds the lower part of the 30S subunit head. Binds mRNA in the 70S ribosome, positioning it for translation (By similarity). Plays a role in mRNA unwinding by the ribosome, possibly by forming part of a processivity clamp. Part of the 30S ribosomal subunit (PubMed:387449, PubMed:7556101, PubMed:12809609, PubMed:16272117, PubMed:27934701, PubMed:10094780, PubMed:12244297, PubMed:27906160, PubMed:27906161, PubMed:28077875). Forms a tight complex with proteins S10 and S14 (By similarity). With proteins S4 and S5 encircles the mRNA as it enters the ribosome, which may play a role in mRNA helicase processivity. Some nascent polypeptide chains are able to cross-link to this protein in situ. Belongs to the universal ribosomal protein uS3 family.