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Summary

The ActA protein of Listeria monocytogenes is a major virulence factor, essential for the recruitment and polymerization of host actin filaments that lead to intracellular motility and cell-to-cell spread of bacteria within the infected host. The expression of actA is tightly regulated and is strongly induced only when L. monocytogenes is within the host cytosol. Intracellular induction of actA expression is mediated through a single promoter element that directs the expression of a messenger RNA with a long (150 bp) 5′ untranslated region (UTR). Deletion of the actA+3 to +130 upstream region was found to result in bacterial mutants that were no longer capable of intracellular actin recruitment or cell-to-cell spread, thus indicating that this region is important for actA expression. L. monocytogenes strains that contained smaller deletions (21–23 bp) within the actA upstream region demonstrated a range of actA expression levels that coincided with the amount of bacterial cell-to-cell spread observed within infected monolayers. A correlation appeared to exist between levels of actA expression and the ability of L. monocytogenes to transition from uniform actin accumulation surrounding individual bacteria (actin clouds) to directional assembly and the formation of actin tails. Bacterial mutants containing deletions that most significantly altered the predicted secondary structure of the actA mRNA 5′ UTR had the largest reductions in actA expression. These results suggest that the actA 5′ UTR is required for maximal ActA synthesis and that a threshold level of ActA synthesis must be achieved to promote the transition from bacteria-associated actin clouds to directional actin assembly and movement.