Expression and control of an operon from an intracellular symbiont which is homologous to the groE operon

Members of the genus Buchnera are intracellular symbionts harbored by the aphid bacteriocyte which selectively synthesize symbionin, a homolog of the Escherichia coli GroEL protein, in vivo. Symbionin and SymS, a GroES homolog, are encoded in the symSL operon, Northern blotting and primer extension analyses revealed that the symSL operon invariably gives rise to a bicistronic mRNA under the control of a heat shock promoter, though the amount of the symSL mRNA in the isolated symbiont did not increase in response to heat shock. The sigma(32) protein that recognizes the heat shock promoter in E. coli was scarcely detected in Buchnera cells even after heat shock. Although the functionally essential regions of the Buchnera sigma 32 protein were well conserved, the Buchnera rpoH gene did not complement an E. coli Delta rpoH mutant. On the one hand, the A-T evolutionary pressure imposed on the Buchnera genome may have not only decreased the activity of its sigma(32) but also ruined the nucleotide sequences necessary for the expression of rpoH; on the other hand, it may have facilitated expression of the symSL operon without activation by sigma(32).