A small heat-shock protein (Hsp20) regulated by RpoS is essential for cyst desiccation resistance in Azotobacter vinelandii.

In Azotobacter vinelandii, a cyst-forming bacterium, the alternative sigma factor RpoS is essential to the formation of cysts resistant to desiccation and to synthesis of the cyst-specific lipids, alkylresorcinols. In this study, we carried out a proteome analysis of vegetative cells and cysts of A. vinelandii strain AEIV and its rpoS mutant derivative AErpoS. This analysis allowed us to identify a small heat-shock protein, Hsp20, as one of the most abundant proteins of cysts regulated by RpoS. Inactivation of hsp20 did not affect the synthesis of alkylresorcinols or the formation of cysts with WT morphology; however, the cysts formed by the hsp20 mutant strain were unable to resist desiccation. We also demonstrated that expression of hsp20 from an RpoS-independent promoter in the AErpoS mutant strain is not enough to restore the phenotype of resistance to desiccation. These results indicate that Hsp20 is essential for the resistance to desiccation of A. vinelandii cysts, probably by preventing the aggregation of proteins caused by the lack of water. To our knowledge, this is the first report of a small heat-shock protein that is essential for desiccation resistance in bacteria.