Enhanced Synthesis of Internalin A inaroMutants ofListeria monocytogenesIndicates Posttranscriptional Control of theinlABmRNA

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Abstract
Listeria monocytogenesmutants with deletions inaroA,aroB, oraroEexhibited 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)-dependentinlABtranscription but was caused by enhanced translation of theinlABtranscripts in thearomutants. AllinlA(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 thearomutants was, however, still observed. Thus, translation of theinlABtranscripts was subject to two modes of posttranscriptional control, one mediated by the UTR structure and the other mediated by thearomutation. The latter mode of control seemed to be related to the predominantly anaerobic metabolism of thearomutants.