Enhanced synthesis of internalin A in aro mutants of Listeria monocytogenes indicates posttranscriptional control of the inlAB mRNA.

Listeria monocytogenes mutants with deletions in aroA, aroB, or aroE exhibited strong posttranscriptional upregulation of internalin A (InlA) and InlB synthesis, which resulted in a more-than-10-fold increase in InlA-mediated internalization by epithelial Caco-2 cells and a 4-fold increase in InlB-mediated internalization by microvascular endothelial cells (human brain microvascular endothelial cells) compared to the wild-type strain. The increase in InlA and InlB production was not due to enhanced PrfA- and/or sigma factor B (SigB)-dependent inlAB transcription but was caused by enhanced translation of the inlAB transcripts in the aro mutants. All inlA(B) transcripts had a 396-nucleotide upstream 5' untranslated region (UTR). Different deletions introduced into this UTR led to significant reductions in InlA and InlB synthesis; enhanced translation of all of the truncated transcripts in the aro mutants was, however, still observed. Thus, translation of the inlAB transcripts was subject to two modes of posttranscriptional control, one mediated by the UTR structure and the other mediated by the aro mutation. The latter mode of control seemed to be related to the predominantly anaerobic metabolism of the aro mutants.