Responsible for synthesis of pseudouridine from uracil at positions 1911, 1915 and 1917 in 23S ribosomal RNA. Isomerization occurs as a late step during the assembly of the large ribosomal subunit. uridine(1911/1915/1917) in 23S rRNA = pseudouridine(1911/1915/1917) in 23S rRNA Belongs to the pseudouridine synthase RluA family.