Caulobacter crescentus is a Gram-negative bacterium, member of the a-subdivision of Proteobacteria that responds to various types of stresses by the expression of alternative sigma factors which permit the rapid adaptation of bacteria to environmental changes. The extracytoplasmic stress response in bacteria is mediated by a special subfamily of sigma factors named ECF that have been classified according to their extra cytoplasmic function. Analysis of the complete genome sequence of C. crescentus has lead to the identification of 13 open reading frames encoding putative ECF sigma factors. In the present work we have investigated the role of ECF sigma factor sigE and a probably co-transcribed gene encoding an EF-hand protein in response to different stresses in C. crescentus. The transmembrane EF-hand protein has 3 EF-hand authentic motifs, suggesting that it is a Calmodulin-like protein. However, Western blot assays showed that a Calmodulin antiserum from the aquatic fungus Blastocladiella emersonii is not able to identify the Caulobacter EF-hand transmembrane protein. C. crescentus sig E and ef-hand genes were isolated by PCR. To investigate the function of the EF-Hand protein, a null mutant (SG246) was obtained by in frame deletion of 100 bp of the ef-hand gene, corresponding to the calcium-binding domain. The ef-hand null mutant was characterized under several stress conditions. SG246 cells were more sensitive than the parental cells (NA1000) to exposure to ethanol (15%). In contrast, the absence of the EF-hand protein makes SG246 cells more resistant than NA1000 cells to exposure to NaCl (150 mM) and high calcium (25 mM). A lacZ transcription fusion with sigE (pRSE) promoter region was constructed and analyzed in SG246 and NA1000 cells by measuring b-galactosidase activity after addition of SDS (0.1%), ethanol (15%), CaCl₂ (2,5 mM), or NaCl (150mM) to both strains. b-galactosidase activity increased only after addition of NaCl (150mM) to SG246 cells. Our results suggest that sigE could play a role in the osmotic stress response generated by NaCl in Caulobacter and that the EF-hand protein may act as a negative regulator of sigE function.