Genome-wide internal tagging of bacterial exported proteins

As a result of the explosive growth of bacterial genomic and postgenomic information, there is a pressing need for efficient, inexpensive strategies for characterizing the in vivo behavior and function of newly identified gene products. We describe here an internal tagging procedure, based on transposon technology, to facilitate the analysis of membrane-bound and secreted proteins in Gram-negative bacteria. The technique is based on a broad host range transposon (ISphoA/hah), which may be used to generate both alkaline phosphatase (AP) gene fusions and 63-codon in-frame insertions in the genome. The 63-codon insertion encodes an influenza hemagglutinin epitope and a hexahistidine sequence, permitting sensitive detection and metal affinity purification of tagged proteins. For each gene targeted, it is thus possible to monitor the disruption of phenotype (using the transposon insertion), the gene's transcription and translation (using the AP reporter activity), and the behavior of the unfused protein (using the internal tag). Studies on a sequence-defined collection of Escherichia coli strains generated using the transposon showed that the synthesis and subcellular localization of tagged proteins could be readily monitored. The use of ISphoA/hah should provide a cost-effective approach for genome-wide in vivo studies of the behavior of exported proteins in a number of bacterial species.