Bacterial lipases from Pseudomonas: regulation of gene expression and mechanisms of secretion

Lipases from Pseudomonas bacteria are widely used for a variety of biotechnological applications. Overexpression in heterologous hosts like Escherichia coli failed to produce enzymatically active lipase prompting to study the molecular mechanisms underlying the regulation of lipase gene expression and secretion. The prototype lipase from P. aeruginosa is encoded in a bicistronic operon which is transcribed from two different promotors, one of which depends on the alternative sigma factor RpoN (sigma(54)). Recently, a two-component regulatory system was identified as an element controlling transcription of the lipase operon. P. aeruginosa lipase is secreted via a type II pathway. The cytoplasmic prelipase contains a 26 amino acid N-terminal signal sequence mediating secretion across the inner membrane via the Sec-machinery. In the periplasm, lipase folds into an enzymatically active conformation assisted by its specific intermolecular chaperone Lif and by unspecific accessory folding catalysts including Dsb-proteins which catalyze the formation of a disulfide bond. Enzymatically active and secretion-competent lipase is finally transported through a complex secretion machinery consisting of 12 different Xcp-proteins of which XcpQ forms a pore-like structure in the outer membrane allowing the release of lipase into the extracellular medium. Biotechnologically important lipases from Burkholderia glumae and P. alcaligenes also use such a type II secretion pathway whereas lipases from P. fluorescens and Serratia marcescens, which lack a typical signal sequence are secreted via a type I pathway. Future challenges to produce Pseudomonas lipases may include artificial up-regulation of lipase gene transcription and construction of more efficient expression strains in which both folding and secretion of lipase are optimized.