ABSTRACT

Buchnera aphidicola is an obligate intracellular symbiont of aphids. One of its proposed functions is the synthesis of essential amino acids, nutrients required by aphids but deficient in their diet of plant phloem sap. The genetic organization of the tryptophan pathway in Buchnera from proliferous aphids of the family Aphididae has previously been shown to reflect a capacity to overproduce this essential amino acid (C.-Y. Lai, L. Baumann, and P. Baumann, Proc. Natl. Acad. Sci. USA 91:3819–3823, 1994). This involved amplification of the genes for the first enzyme in the pathway, anthranilate synthase (TrpEG), on a low-copy-number plasmid. Here we report on the finding and molecular characterization of TrpEG-encoding plasmids in Buchnera from aphids of the distantly related family Pemphigidae. Buchnera fromTetraneura caerulescens contained a 3.0-kb plasmid (pBTc2) that carried a single copy of trpEG and resembledtrpEG plasmids of Buchnera from the Aphididae. The second plasmid (pBPs2), isolated from Buchnera ofPemphigus spyrothecae, contained a different replicon. It consisted of a putative origin of replication containing iterons and an open reading frame, designated repAC, which showed a high similarity to the gene encoding the replication initiation protein RepA of the RepA/C replicon from the broad-host-range IncA/C group of plasmids. The plasmid population was heterogeneous with respect to the number of tandem repeats of a 1.8-kb unit carryingrepAC₁ , trpG, and remnants of trpE. The two principal forms consisted of either five or six copies of this repeat and a single-copy region carryingrepAC₂ , the putative origin of replication, andtrpE. The unexpected finding of elements of the RepA/C replicon in previously characterized trpEG plasmids fromBuchnera of the Aphididae suggests that a replacement of replicons has occurred during the evolution of these plasmids, which may point to a common ancestry for all Buchnera trpEGamplifications.