Plasmid-encoded anthranilate synthase (TrpEG) in Buchnera aphidicola from aphids of the family pemphigidae

Buchnera aphidicola is an obligate intracellular symbiont of aphids, One of its proposed functions is the synthesis of essential amino acids, nutrient s required by aphids but deficient in their diet of plant phloem sap. The g enetic organization of the tryptophan pathway in Buchnera from proliferous aphids of the family Aphididae has previously been shown to reflect a capac ity 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 amp lification of the genes for the first enzyme in the pathway, anthranilate s ynthase (TrpEG), on a low-copy-number plasmid. Here we report on the findin g and molecular characterization of TrpEG-encoding plasmids in Buchnera fro m aphids of the distantly related family Pemphigidae, Buchnera from Tetrane ura caerulescens contained a 3.0-kb plasmid (pBTc2) that carried a single c opy of trpEG and resembled trpEG plasmids of Buchnera from the Aphididae, T he second plasmid (pBPs2), isolated from Buchnera of Pemphigus spyrothecae, contained a different replicon, It consisted of a putative origin of repli cation containing iterons and an open reading frame, designated repAC, whic h 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 n umber of tandem repeats of a 1.8-kb unit carrying repAC(1), trpG, and remna nts of trpE, The two principal forms consisted of either five or six copies of this repeat and a single-copy region carrying repAC(2), the putative or igin of replication, and trpE, The unexpected finding of elements of the Re pA/C replicon in previously characterized trpEG plasmids from Buchnera of t he Aphididae suggests that a replacement of replicons has occurred during t he evolution of these plasmids, which may point to a common ancestry for al l Buchnera trpEG amplifications.