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Expression of the Listeria monocytogenes EGD inlA and inlB genes, whose products mediate bacterial entry into tissue culture cell lines, by PrfA-dependent and -independent mechanisms

Internalization of Listeria monocytogenes into nonphagocytic cell lines in vitro requires the products of the inlAB locus (J.-L. Gaillard, P. Berche, C. Frehel, E. Gouin, and P. Cossart, Cell 65:1127-1141, 1991). By generating isogenic mutants with a chromosomal in-frame deletion in either inlA or inlB, we have identified InlA and InlB as surface-bound proteins of L. monocytogenes with molecular weights of 88,000 and 65,000, respectively. These results were obtained with monoclonal antibodies raised against either protein and corroborated by N-terminal end sequencing of InlA and InlB. By immunoblot analysis, the production of both polypeptides was found to be strongly dependent on growth temperature and, particularly for InlB, on the presence of the PrfA regulator protein. Expression of InlA was not strictly dependent on the presence of the PrfA regulator protein. Transcription analysis of the inlAB locus revealed that the inlA gene was transcribed by several promoters, of which only one is PrfA dependent. This PrfA-dependent inlA promoter, which contains two base substitutions within its putative PrfA DNA-binding palindrome, is responsible for transcription of both inlA and inlB genes. A hitherto unrecognized promoter located 51 bp upstream of the GTG start codon of the inlB gene was also detected. Hence, inlA and inlB are transcribed both individually and in an operon by PrfA-dependent and -independent mechanisms. Tissue culture invasion assays employing various epithelial cell lines demonstrated that both InlA and InlB are required for invasion. In vivo studies using the mouse infection model revealed that both internalin mutants were attenuated for virulence.