Responsible for synthesis of pseudouridine from uracil-55 in the psi GC loop of transfer RNAs. Also catalyzes pseudouridylation of mRNAs with the consensus sequence 5'-GGUUCRA-3'. uridine(55) in tRNA = pseudouridine(55) in tRNA a uridine in mRNA = a pseudouridine in mRNA Belongs to the pseudouridine synthase TruB family.