Spot 42 sRNA regulates arabinose-inducible araBAD promoter activity by repressing synthesis of the high-affinity low-capacity arabinose transporter.

The L-arabinose-inducible araBAD promoter (PBAD) allows tightly controlled and tunable expression of genes of interest in a broad range of bacterial species. It has been successfully used to study bacterial sRNA regulation, where PBAD drives expression of target mRNA translational fusions. Here we report that in Escherichia coli, Spot 42 sRNA can regulate PBAD promoter activity by affecting arabinose uptake. We demonstrate that Spot 42 sRNA represses araF, a gene encoding the AraF subunit of the high-affinity low-capacity arabinose transporter AraFGH, through direct base pairing interactions. We further show that endogenous Spot 42 sRNA is sufficient to repress araF expression under various growth conditions. Finally, we demonstrate this posttranscriptional repression has a biological consequence, decreasing the induction of PBAD at low levels of arabinose. This problem can be circumvented using strategies reported previously for avoiding all-or-none induction behavior, that is through constitutive expression of the low-affinity high-capacity arabinose transporter AraE or induction with higher concentration of inducers. This work adds araF to the set of Spot 42-regulated genes, in agreement with previous studies suggesting that Spot 42, itself negatively regulated by cAMP-CRP complex, reinforces the catabolite repression network.

IMPORTANCE: The bacterial arabinose inducible system is widely used for titratable control of gene expression. We demonstrate here that a post-transcriptional mechanism mediated by the Spot 42 sRNA contributes to the functionality of the PBAD system at subsaturating inducer concentrations by affecting inducer uptake. Our finding extends the inputs into the known transcriptional control for the PBAD system, and has implications for improving its usage for tunable gene expression.