Like other bacteria, Bacillus subtilis possesses a family of homologous small acidic proteins (CspB, CspC and CspD, identity >70%) that are strongly induced in response to cold shock. We show that deletion of cspC or cspD genes did not result in a detectable phenotype; in contrast, csp double mutants exhibited severe reduction in cellular growth at 15 degrees C as well as at 37 degrees C, including impairment of survival during the stationary phase. Two-dimensional gel analysis showed that protein synthesis was deregulated in csp double mutants and that the loss of one or two CSPs led to an increase in the synthesis of the remaining CSP(s) at 37 degrees C and after cold shock, suggesting that CSPs down-regulate production of members from this protein family. A cspB/C/D triple mutant (64BCDbt) could only be generated in the presence of cspB in trans on a plasmid that was not lost, in spite of lack of antibiotic pressure, indicating that a minimum of one csp gene is essential for viability of B. subtilis. After cold shock, synthesis of CspB in 64BCDbt was drastically lower than in wild-type cells accompanied by cessation in growth and strong reduction in general protein synthesis. As CspB, CspC and CspD are shown to bind to RNA in a co-operative and interactive manner, CSPs are suggested to function as RNA chaperones facilitating the initiation of translation under optimal and low temperatures.