Structure of the dnaA region of the endosymbiont, Buchnera aphidicola, of aphid Schizaphis graminum.

Buchnera aphidicola is an intracellular prokaryote (endosymbiont) that lives in the body cavity of the aphid. Phylogenetic studies indicated that it is closely related to Escherichia coli and members of Enterobacteria. The gene order of the region containing the dnaA gene is well conserved in many bacteria. Seven genes of the endosymbiont of the aphid Schizaphis graminum, gyrB, dnaN, dnaA, rpmH, rnpA, yidD, and 60K. were found to be homologous in sequence and relative location to those of E. coli. We have further sequenced the region downstream of the 60K gene to elucidate the boundary of the conserved region, and found that one more gene, thdF, is conserved. The comparison of gene organizations of the dnaA region of the related bacteria supported the close phylogenetic relationship of B. aphidicola to E. coli. In addition, we have identified groES and groEL genes next to the thdF gene. GroEL protein was reported to be expressed at an elevated level in the endosymbionts of aphids, and is considered to play an important role in their association with the aphid host. Comparison of the structure of the groE operon with that of the endosymbiont of the aphid Acyrthosiphon pisum revealed the conservation of a sequence resembling the E. coli consensus heat shock promoter, and this sequence may be responsible for the high expression of the groEL gene in aphid endosymbionts.