When tested in a heterologous in vitro system, purified recombinant B. subtilis RluB (YpuL) produces pseudouridine at position 552 in E.coli 16S rRNA and at positions 1199, 2605, and 2833 in E. coli 23S rRNA. Of these, only pseudouridine at position 2605 (position 2633 in B. subtilis 23S rRNA) is found naturally in 23S rRNA of both B. subtilis and E. coli. For this reason, B. subtilis RluB has been considered as the homolog of E. coli RluB. Notice, though B. subtilis RluB displays 41% sequence similarity with E.coli RsuA (Y516), it does not catalyze the pseudouridine formation at the conserved U in B. subtilis 23S rRNA corresponding to Y516 in E. coli 23S rRNA. Therefore, at variance with site-specific E. coli RluB (Y2605), the B. subtilis homolog displays an in vitro multi-site specificity at other sites than those normally modified in rRNA of E. coli. The in vivo specificity of B. subtilis RluB has not yet been experimentally verified (March 2012).