Annotation of sequences of transposon insertion sites in the chromosome of Escherichia coli O157:H7 cells that had lost their acid-resistance properties revealed four genes: two encoding isoforms of glutamate decarboxylase (GadA and GadB) and one encoding a glutamate/gamma-aminobutyric acid (GABA) antiporter. What does the fourth gene encode?
A) a heat shock protein B) a possible regulator C) a glutamate/GABA symporter D) a GABA uniporter E) a glutamate carrier protein