A gene fusion approach to the study of pullulanase export and secretion in Escherichia coli

A series of fusions between the gene for the Klebsiella pneumoniae secreted lipoprotein pullulanase (pulA) and the genes for cytoplasmic beta-galactosidase (lacZ) or periplasmic alkaline phosphatase (phoA) were created by transposon mutagenesis using mini-MudII1681 or TnphoA, respectively. The hybrid genes were expressed in Escherichia coli K-12 with or without the K. pneumoniae genes that promote pullulanase secretion in E. coli. We characterized seven different pulA-lacZ gene fusions encoding hybrid polypeptides containing from 14 to c. 1060 residues of pro-pullulanase. All but the smallest hybrid were fatty acylated and were toxic to producing cells, causing the accumulation of precursors of other exported proteins. Four different pulA-phoA gene fusions encoded hybrids with alkaline phosphatase activity. All four hybrids were fatty acylated, but were not toxic. Although the hybrids were apparently membrane-associated, they were not secreted into the medium either by E. coli carrying pullulanase secretion genes or by K. pneumoniae. Immunofluorescence tests indicated that the pullulanase secretion genes promoted the localization of one of these hybrids to the outer face of the E. coli outer membrane, which may have important implications for the design of live vaccine strains and of immobilized enzymes.