Molecular characterization of the leucine plasmid from Buchnera aphidicola, primary endosymbiont of the aphid Acyrthosiphon pisum

The complete sequence of the leucine plasmid of Buchnera aphidicola from th e aphid Acyrthosiphon pisum (pLeu-BAp) is reported. Its gene organization w as concordant with those of ether leucine plasmids of Buchnera from aphids of the Aphidini and Macrosiphini tribes. Three inverted repeats are present in pLeu-BAp. Two of them are also present in pLeu from the family Aphidida e: (i) SIR1, located downstream the leucine operon, resembles a rho-indepen dent terminator of transcription, and (ii) LIR1, located upstream of the le ucine operon, is suggested to be involved in transcription termination of m essenger stability. The third, located near the putative ATGC repeats invol ved in the origin of replication, is specific in aphids of the Macrosiphini tribe. Phylogenetic analyses based on sequences of leuA, leuB, leuC, leuD, repA1 and ORF1 showed a closer relationship between Buchnera (A. pisum) an d Bucknera (Diuraphis noxia). However, tree topologies indicate that the sp lit between both aphid species took place soon after the formation of the M acrosiphini lineage.