pstS encodes the soluble periplasmic phosphate-binding protein of the E. coli
PstSACB high-affinity ABC importer. The experimentally direct molecular
function is phosphate binding: Luecke and Quiocho solved a 1.7 A liganded
structure and used it to explain the hydrogen-bonding basis of phosphate
selectivity PMID:2215649.
PstS should be curated as the periplasmic capture subunit of the assembled
substrate-binding-protein-dependent ABC importer, not as an integral membrane
protein. Surin et al. found that phoS encodes a 25-residue signal peptide
followed by a mature 321-residue periplasmic phosphate-binding protein
PMID:6321434. The later pst-region sequencing paper placed the operon in the
order phoS pstC pstA pstB phoU, tying PstS genetically to the core Pst
transport machinery PMID:3881386.
The DNA-damage and radiation-response annotations look secondary. Khil and
Camerini-Otero measured expression changes after mitomycin C for nearly 30% of
all E. coli genes PMID:11967071, and the radiation row comes from a
whole-Keio deletion sensitivity screen PMID:27718375. Those are useful
phenotypes but do not change the core function away from phosphate capture and
delivery to PstSACB.