In recent years, the emergence of antimicrobial resistance in previously susceptible common pathogenic bacteria has been a major concern in public health. Staphylococcus aureus
is amongst the most significant of such pathogens; it is found widely in hospitals and, if untreated, can lead to death through sepsis. Methicillin-resistant S. aureus
(MRSA) demonstrates resistance not only to methicillin but also to other penicillin-type antibiotics (Foster, 1996
). MRSA is now widespread in UK hospitals (Johnson et al.
) and is termed a ‘superbug’ by the popular press. In addition to its presence in hospitals, community-acquired MRSA strains are now being reported around the world (Holden et al.
; Pesavento et al.
; Boyle-Vavra & Daum, 2007
). The increase in the rate of methicillin resistance was slow and gradual in the UK during the 1980s. The subsequent surge in infections has been attributed in part to community-based infections taking hold (Herold et al.
). Many such community cases occur in otherwise healthy individuals with few risk factors for MRSA (Herold et al.
) and the infections are often centred on wounds. The resistance of MRSA to β-lactams arises from a number of changes: impaired β-lactam entry into bacteria, instability of β-lactams to bacterial serine- or metallo-β-lactamases or the loss of β-lactam binding affinity to penicillin-binding protein (Edwards et al.
; Bush et al.
). Finding novel targets to treat staphylococcal infections, particularly MRSA, is a pressing problem. We have used two-dimensional gel proteomics to identify expression differences between MRSA strain 252 and the methicillin-sensitive S. aureus
strain 476 (MSSA). We have identified that the expression of Sar2028 is upregulated in MRSA252 compared with MSSA476. MRSA252 Sar2028 is an acidic protein (MW 48 168 Da; pI 5.25; Holden et al.
) previously annotated as a hypothetical protein belonging to the pyridoxal 5′-phosphate (PLP) dependent aminotransferase superfamily.
Aminotransferases are ubiquitous metabolic enzymes that catalyze the transfer of an amino group from an amino acid to either a keto acid or an aldehyde. PLP, an active form of vitamin B6
, is the essential cofactor for catalysis. During the aminotransferase reaction, PLP serves as a transitory acceptor of the amino group from the donor substrate and is converted to pyridoxamine 5′-phosphate (PMP). PMP then serves as a donor of the amino group to the acceptor substrate, with accompanying regeneration of PLP (Hirotsu et al.
). Most PLP-dependent enzymes contain a conserved lysine residue which forms a Schiff base with the coenzyme in the resting state; the species is referred to as the internal aldimine. Sar2028 has relatively low sequence homology (<25%) to known PLP enzymes and its function is not known. At a 60% identity threshold, homologues of the protein can be identified in several other organisms, including Bacillus
spp. As part of our program on MRSA, we have cloned, expressed, purified and crystallized this protein.