Loss of CsrA in Acinetobacter baumannii leads to lethal dysregulation of alanine metabolism

Posttranscriptional regulation is a key mechanism by which bacteria coordinate gene expression in response to changing environments. In the human pathogen Acinetobacter baumannii , the importance of posttranscriptional gene expression control is exemplified by CsrA, a highly conserved posttranscriptional regulator that is essential for viability in multiple settings, ranging from growth in rich laboratory media to infection-relevant conditions like human serum. However, csrA is dispensable for growth in chemically defined media with a single carbon source. We leveraged this condition-specific essentiality to identify suppressor mutations that regained the ability to grow in rich media. These analyses identified Lrp, a transcription factor that regulates hundreds of genes in A. baumannii. Among the Lrp regulon, we identified a four gene locus involved with uptake and utilization of D-alanine. We determined that two transcripts encoded by this locus, dadA and cycA2 , are repressed by CsrA. The absence of csrA leads to overexpression of both dadA and cycA2 , which is toxic, even in wild type A. baumannii . In the csrA mutant, this dysregulation contributes to impaired cell envelope integrity and increased sensitivity to several clinically relevant cell-wall targeting antibiotics, including carbapenems. Collectively, our data indicate that Lrp and CsrA coordinately balance expression of D-alanine utilization genes in A. baumannii , at the transcriptional and posttranscriptional levels, respectively. Moreover, it appears that a key aspect of CsrA’s essentiality in A. baumannii is to counter the activities of Lrp, a second global regulator. These findings further highlight the complex interplay between transcriptional and posttranscriptional regulators in bacteria.