DISCOVERY AND MOLECULAR CHARACTERIZATION OF A PLASMID LOCALIZED IN BUCHNERA SP BACTERIAL ENDOSYMBIONT OF THE APHID RHOPALOSIPHUM PADI

We have identified and completely sequenced a novel plasmid isolated f rom the aphid Rhopalosiphum padi. Evidence which suggests that the pla smid occurs localized within the bacterial endosymbionts is presented. The plasmid contains the four genes that constitute the entire leucin e operon. This fact makes it really unique since most plasmids are dis pensable and lack genes that encode essential anabolic functions. Four more phloem-feeding aphid species also seem to contain homologous pla smids. Although further work is necessary, we hypothesize that this pl asmid has appeared during the evolution of the symbiotic association b etween the aphid and the bacterial endosymbiont. The fact that this pl asmid contains the entire leucine operon can be related to physiologic al evidence showing that the aphid host' a diet of plant phloem is def icient in essential amino acids.