Buchnera plasmid-associated trpEG probably originated from a chromosomal location between hsIU and fpr

Buchnera are prokaryotic endosymbionts found in most aphids. One of their f unctions is the synthesis of the essential amino acid tryptophan for the ap hid host. In Buchnera from some aphids that have a long development time, t rpEG, which encodes the first enzyme of the tryptophan biosynthetic pathway (anthranilate synthase), is found as one copy on the endosymbiont chromoso me and is located between hslU and fpr. In Buchnera from Schizaphis graminu m, which has a short development time, trpEG is amplified on plasmids. We h ave cloned and sequenced a 4.1-kb DNA fragment from Buchnera of S. graminum and have found the gene order hsIU-ibp-fpr-yjeA-kdtB. The proximity of hsl U and fpr is consistent with the excision, in an endosymbiont ancestor, of trpEG from a location between these two genes, with the excision either fol lowed or preceded by acquisition of ibp.