This is one of the proteins that binds and probably mediates the attachment of the 5S RNA into the large ribosomal subunit, where it forms part of the central protuberance. Part of the 50S ribosomal subunit; part of the 5S rRNA/L5/L18 subcomplex. In B.stearothermophilus only 2 proteins, L5 and L18 have been shown to be part of this subcomplex, unlike the case in E.coli and T.thermophilus where L25 (TL5) is also found. The protein, when overexpressed in E.coli, contains a phosphoserine, which is required for the protein to bind to 5S rRNA (PubMed:10529214). It has been suggested, based solely on amino acid conservation, that this occurs on Ser-57. Belongs to the universal ribosomal protein uL18 family.