Genetic characterization of plasmids containing genes encoding enzymes of leucine biosynthesis in endosymbionts (Buchnera) of aphids

The prokaryotic endosymbionts (Buchnera) of aphids are known to provision t heir hosts with amino acids that are limiting in the aphid diet. Buchnera f rom the aphids Schizaphis graminum and Diuraphis nox-ia have plasmids conta ining leuABCD, genes that encode enzymes of the leucine biosynthetic pathwa y, as well as genes encoding proteins probably involved in plasmid replicat ion (repA1 and repA2) and an open reading frame (ORF1) of unknown function. The newly reported plasmids closely resemble a plasmid previously describe d in Buchnera of the aphid Rhopalosiphum padi [Bracho AM, Martinez-Torres D , Moya A, Latorre A (1995) J Mol Evol 41:67-73]. Nucleotide sequence compar isons indicate conserved regions which may correspond to an origin of repli cation and two promoters, as well as inverted repeats, one of which resembl es a rho-independent terminator. Phylogenetic analyses based on amino acid sequences of leu gene products and ORF1 resulted in trees identical to thos e obtained from endosymbiont chromosomal genes and the plasmid-borne trpEG. These results are consistent with a single evolutionary origin of the leuA BCD-containing plasmid in a common ancestor of Aphididae and the lack of pl asmid exchange between endosymbionts of different aphid species. Trees for ORF1 and repA (based on both nucleotides and amino acids) are used to exami ne the basis for leu plasmid differences between Buchnera of Thelaxes suber i and Aphididae. The most plausible explanation is that a single transfer o f the leu genes to an ancestral replicon was followed by rearrangements. Th e related replicon in Buchnera of Pemphigidae, which lacks leuABCD, appears to represent the ancestral condition, implying that the plasmid location o f the leu genes arose after the Pemphigidae diverged from other aphid famil ies. This conclusion parallels previously published observations for the un related trpEG plasmid, which is present in Aphididae and absent in Pemphigi dae. Recruitment of amino acid biosynthetic genes to plasmids has been ongo ing in Buchnera lineages after the infection of aphid hosts.