Type I signal
peptidases (SPases) cleave
signal peptides from
proteins during translocation across
biological membranes and hence play a vital role in cellular physiology.
SPase activity is also of fundamental importance to the pathogenesis of
infection for many bacteria, including Pseudomonas aeruginosa, which utilizes a variety of secreted
virulence factors, such as
proteases and toxins. P. aeruginosa possesses two noncontiguous
SPase homologues, LepB (PA0768) and PA1303, which share 43%
amino acid identity. Reverse transcription (RT)-PCR showed that both
proteases were expressed, while a FRET-based assay using a
peptide based on the
signal sequence cleavage region of the secreted LasB
elastase showed that recombinant LepB and PA1303
enzymes were both active. LepB is positioned within a genetic locus that resembles the locus containing the extensively characterized
SPase of E. coli and is of similar size and topology. It was also shown to be essential for viability and to have high sequence identity with SPases from other pseudomonads (≥ 78%). In contrast, PA1303, which is small for a Gram-negative
SPase (20 kDa), was found to be dispensable. Mutation of PA1303 resulted in an altered
protein secretion profile and increased N-butanoyl
homoserine lactone production and influenced several quorum-sensing-controlled phenotypic traits, including swarming motility and the production of
rhamnolipid and elastinolytic activity. The data indicate different cellular roles for these P. aeruginosa
SPase paralogues; the role of PA1303 is integrated with the quorum-sensing cascade and includes the suppression of
virulence factor secretion and virulence-associated phenotypes, while LepB is the primary
SPase.