Caulobacter crescentus CdnL is a non-essential RNA polymerase-binding protein whose depletion impairs normal growth and rRNA transcription.

CdnL is an essential RNA polymerase (RNAP)-binding activator of rRNA transcription in mycobacteria and myxobacteria but reportedly not in Bacillus. Whether its function and mode of action are conserved in other bacteria thus remains unclear. Because virtually all alphaproteobacteria have a CdnL homolog and none of these have been characterized, we studied the homolog (CdnL) of the model alphaproteobacterium Caulobacter crescentus. We show that CdnL is not essential for viability but that its absence or depletion causes slow growth and cell filamentation. CdnL is degraded in vivo in a manner dependent on its C-terminus, yet excess CdnL resulting from its stabilization did not adversely affect growth. We find that CdnL interacts with itself and with the RNAP β subunit, and localizes to at least one rRNA promoter in vivo, whose activity diminishes upon depletion of CdnL. Interestingly, cells expressing CdnL mutants unable to interact with the RNAP were cold-sensitive, suggesting that CdnL interaction with RNAP is especially required at lower than standard growth temperatures in C. crescentus. Our study indicates that despite limited sequence similarities and regulatory differences compared to its myco/myxobacterial homologs, CdnL may share similar biological functions, since it affects rRNA synthesis, probably by stabilizing open promoter-RNAP complexes.