In contrast, little is known about C-terminal processing of proteins in prokaryotes (Menon et al., 1993; Rossmann et al., 1994; Aceto et al., 1999; Hatchikian et al., 1999; Keiler & Sauer, 2004). The CTP are classified in the MEROPS peptidase database as family S41 (http://merops.sanger.ac.uk) (Rawlings et al., 2008). CTPs can be found in a broad range of different organisms, for example in prokaryotes such as Eubacteria and Archaea, as well as eukaryotes, for example algae, plants and animals (Inagaki & Satoh, 2004; Keiler & Sauer, 2004; Tamura & Baumeister, 2004). In plants, algae and cyanobacteria CTPs have a very specific function in activating the pre-D1

protein by cleaving a small C-terminal peptide (Trost et al., 1997; Fabbri et al., 2005). The mature D1 protein is an important constituent of the photosystem II reaction centre and its processing is essential for photosynthesis and thus for the viability

find more of these organisms under phototrophic conditions (Satoh & Yamamoto, 2007). Compared with this, the knowledge on bacterial CTPs is extremely limited. The first bacterial CTP that was characterized was the ‘Tail-specific protease’ (Tsp), which was purified from Escherichia coli and showed activity in degrading protein variants with nonpolar C-termini of the λ repressor (Silber et al., 1992). Tsp, more commonly referred to as Prc – is also involved in processing of penicillin-binding protein-3 (PBP-3), by cleaving 11 C-terminal amino acids (Hara et al., 1991) and interacting with lipoprotein NlpI (Tadokoro et al., 2004). Besides that, Prc has been suggested to be part of the SsrA RNA small molecule library screening protein-tagging system for the degradation of incorrectly synthesized proteins. In this system, an SsrA RNA tag is added to mRNAs when ribosomes are stalled due to a lack of termination codons. The resulting C-terminal SsrA peptide tagged periplasmic protein is then recognized by Prc and subsequently degraded (Keiler et al., 1996). CTP-inactivated bacterial mutants show different phenotypes. In E. coli,

inactivation of the prc gene results in leakage of periplasmic proteins, temperature-sensitive very growth under osmotic stress, reduced heat-shock response and increased antibiotic susceptibility (Hara et al., 1991; Seoane et al., 1992). Inactivation of ctpA in Rhizobium leguminosarum led to a decreased desiccation tolerance (Gilbert et al., 2007). Recently, inactivation of CTP was shown to influence the pathogenesis of several Gram-negative bacteria, Brucella suis, Bartonella bacilliformis, Chlamydia trachomatis and Burkholderia mallei (Mitchell & Minnick, 1997; Bandara et al., 2005, 2008; Lad et al., 2007). CTPs seem to influence multiple basal physiological functions in bacteria. The knowledge of their subcellular localization would enable a much better understanding in how these proteases interact and influence other cellular systems.