Pea enation mosaic virus (PEMV) is composed of two autonomously replicating virus RNAs related to the genomic RNAs of viruses in the generaLuteovirus and Umbravirus. The transmission of PEMV resembles that of its luteovirus relatives in utilizing circulative aphid transmission. However, unlike its luteovirus counterparts, PEMV can also be mechanically transmitted. Prolonged mechanical passage of PEMV can lead to the loss of aphid transmissibility, a trait that is mirrored by specific changes in the PEMV virion composition. These changes were used to examine the virus contribution to vector transmission and the mechanisms by which it is regulated. Using a local lesion isolation technique, one aphid transmissible and two aphid non-transmissible isolates of PEMV were compared. Structural analysis of a 54 kDa minor structural subunit unique to the aphid transmissible isolate