... fluorogenic synthetic peptide substrate. These obser-vations highlight the importance of the amino acids in the transmembranesegment and also those preceding the catalytic serine in the sequence of SpsB. ... in the case of LepB,SipS and Spi. A comparison of the site of cleavage of SpsB with that of Spi from S. pneumoniae showsthat they are cleaved at the same point, whereas, in the case of SipS from ... essentiality, the eas-ier accessibility of the catalytic domain for potentialinhibitors as a result of being located on the outer side of the cytoplasmic membrane, and the different cata-lytic mechanism...