A LysR-type transcriptional regulator controls the expression of numerous small RNAs in Agrobacterium tumefaciens.

Small RNAs (sRNAs) are universal posttranscriptional regulators of gene expression and hundreds of sRNAs are frequently found in each and every bacterium. In order to coordinate cellular processes in response to ambient conditions, many sRNAs are differentially expressed. Here, we asked how these small regulators are regulated using Agrobacterium tumefaciens as a model system. Among the best-studied sRNAs in this plant pathogen are AbcR1 regulating numerous ABC transporters and PmaR, a regulator of peptidoglycan biosynthesis, motility, and ampicillin resistance. We report that the LysR-type regulator VtlR (also known as LsrB) controls expression of AbcR1 and PmaR. A vtlR/lsrB deletion strain showed growth defects, was sensitive to antibiotics and severely compromised in plant tumor formation. Transcriptome profiling by RNA-sequencing revealed more than 1,200 genes with altered expression in the mutant. Consistent with the function of VtlR/LsrB as regulator of AbcR1, many ABC transporter genes were affected. Interestingly, the transcription factor did not only control the expression of AbcR1 and PmaR. In the mutant, 102 sRNA genes were significantly up- or downregulated. Thus, our study uncovered VtlR/LsrB as the master regulator of numerous sRNAs. Thereby, the transcriptional regulator harnesses the regulatory power of sRNAs to orchestrate the expression of distinct sub-regulons.