As a result of the nucleotide sequence analysis of an aphid endosymbiont's 
operon homologous to the Escherichia coli groE, we noted that directional base 
substitutions tending toward an increase of A + T content represent an obvious 
evolutionary trend in this prokaryotic operon, housed for a long period by an 
eukaryotic cell. This result, when taken together with previous reports, raised 
the possibility that genomic DNA of prokaryotes residing in an eukaryotic cell 
is subject to A/T-biased directional mutation pressure and/or both negative and 
positive selection operating under conditions specific to the intracellular 
environments.