A dual-function regulatory element couples ParB expression and DNA substrate specificity

Chromosome segregation is essential for cell survival. Most bacteria encode the chromosome partitioning ParABS system. Although even small changes in ParA or ParB levels disrupt genome maintenance, the mechanisms that control their abundance have remained unresolved. Using Caulobacter crescentus, we provide the first mechanistic evidence that ParB levels are regulated post-transcriptionally. Through single-nucleotide substitutions and compensatory mutation analyses, we identify an mRNA secondary structure at the 5 end of the parB transcript that enhances ParB cellular abundance. Loss of this regulatory mechanism sensitizes cells to modest increases in ParA levels, causing cell death. Additionally, we demonstrate that ParB substrate specificity is not determined solely by the central helix-turn-helix domain that interacts with parS but is unexpectedly modulated by the N-terminal domain. Together, our findings reveal an uncharacterized layer of chromosome segregation control and highlight post-transcriptional regulation of ParB abundance as a potential mechanism for maintaining the precise ParA-ParB balance required for bacterial viability.