hspA encodes a small heat shock protein (sHSP) in Xanthomonas campestris pv. campestris, the causative agent of black rot in cruciferous plants. In this study, two-dimensional gel electrophoresis, promoter activity assays, and Northern hybridization results revealed that HspA expression was induced by heat shock but not by other stresses, although low-level expression was detectable by reverse transcription-polymerase chain reaction (RT-PCR) under normal culture conditions. An hspA mutant exhibited reduced tolerance to heat, especially in the presence of MgSO4, but no change in pathogenicity. Results of size-exclusion chromatography indicated that purified HspA(his), containing six C-terminal histidine residues, formed two different size classes of oligomeric complexes--410 and 820 kDa. In contrast, HspA(ter), the unmodified protein translated from the original hspA gene, formed only the 820-kDa complex. These results suggest that the C-terminus of HspA is important for oligomerization. Both HspA820(his) and HspA410(his) were able to partially protect luciferase against heat-induced aggregation. Unlike other reported sHSPs that commonly capture denaturing proteins in refoldable states until refolded by adenosine triphosphate-dependent chaperone systems, HspA(his) alone was capable of reactivating heat-inactivated EcoRI. Thus, Xanthomonas campestris pv. campestris HspA has potential application as a protective agent during the storage of proteins.