Multiple Small Heat Shock Proteins in Rhizobia

Seven genes coding for small heat shock proteins (sHsps) in Bradyrhizobium japonicum have been identified. They are organized in five operons that are coordinately regulated by ROSE, a negatively cis-acting DNA element. The deduced sHsps can be divided into two separate classes: class A, consisting of proteins that show similarity to Escherichia coli IbpA and IbpB, and class B, whose members display significant similarity to other sHsps from prokaryotes and eukaryotes. Two-dimensional gel electrophoresis and Edman sequencing revealed the presence of at least 12 sHsps in B. japonicum, indicating a remarkable abundance of sHsps in this organism. Three additional members of class A and two potentially novel heat shock proteins were identified on the basis of their amino termini. The presence of multiple sHsps was also demonstrated for a variety of Rhizobium and Bradyrhizobium species by immunoblot analysis and two-dimensional gel electrophoresis. An extensive database survey revealed that, in contrast to the rhizobia, other bacteria contain maximally two sHsps whereas many plants have been reported to possess a sHsp superfamily.