Pseudomonas (P.) aeruginosa is a clinically significant opportunistic human pathogen and the major cause of morbidity and mortality in cystic fibrosis (CF) patients. Persistence of P. aeruginosa in the CF lung is facilitated by processes such as biofilm formation and antibiotic resistance. ATP-binding cassette (ABC) transporters are ubiquitous and are often implicated in multidrug efflux. In prokaryotes such as P. aeruginosa, they exhibit a diverse range of functions. The P. aeruginosa PAO1 genome encodes 4 uncharacterised putative ABC transporters that have canonical efflux architecture. We have investigated the physiological role of PA1876, a predicted ABC transporter located within an operon with a putative membrane fusion protein (PA1877), outer membrane protein (PA1875), and large secreted protein (PA1874). This genetic locus suggests that PA1876 is the ABC transporter component of a Type I secretion system (TISS).

We have isolated PA1876, the ABC transporter component of the proposed TISS, to elucidate its function. Recombinant PA1876 was solubilised from E. coli membranes in the detergent Fos-choline 14. Purified PA1876 had a relative molecular mass of 163 kDa under non-denaturing electrophoretic conditions, consistent with a native homodimeric state of the protein. Our investigation into the function of the proposed PA1874-PA1877 TISS found that it had a major role in biofilm formation. We show that an unmarked P. aeruginosa PAO1 ΔPA1875-PA1877 mutant strain had a >50% reduction in biofilm formation relative to the wild type strain. Taken together these data indicate that PA1876 is the ABC transporter component of a TISS system involved in biofilm formation.