Arch Virol (2001) 146: 1607–1615 Brome mosaic virus replicase proteins localize with the movement protein at infection-specific cytoplasmic inclusions in infected barley leaf cells Brief Report K. Dohi, M. Mori ∗ , I. Furusawa, K. Mise, and T. Okuno Laboratory of Plant Pathology, Graduate School of Agriculture, Kyoto University, Kyoto, Japan Accepted May 29, 2001 Summary. Subcellular localization of the Brome mosaic virus replicase-related 1a and 2a proteins, and the 3a movement protein in infected barley leaves was examined by immunogold electron microscopy. The 1a and 2a proteins colocal- ized at infection-specific electron-dense cytoplasmic inclusions. The 3a protein was also detected in these inclusions. The inclusions were oval or amorphous, and contained electron-lucent regions. Electron microscopic autoradiography showed that the inclusions were sites of [ 3 H]uridine incorporation, suggesting that they are sites of viral RNA synthesis. * Brome mosaic virus (BMV) is a non-enveloped icosahedral positive-stranded RNA plant virus, belonging to the genus Bromovirus in the family Bromoviri- dae, which is included in the ‘alphavirus-like’ superfamily [1, 2]. The BMV genome is composed of three RNAs designated RNA1, RNA2, and RNA3 [10]. RNA1 encodes the 109-kDa 1a protein which contains an N-terminal m 7 G- methyltransferase-like domain and a C-terminal helicase-like domain, and RNA2 encodes the 94-kDa 2a protein which has sequence similarities to the RNA- dependent RNA polymerases (RdRps) [1]. The 1a and 2a proteins are thought to comprise the viral RdRp complex together with some host proteins, including eukaryotic elongation initiation factor 3 or a related protein [5, 16–18]. RNA3 encodes the 32-kDa 3a protein, and the 20-kDa coat protein which is trans- ∗ Present address: Laboratory of Plant Molecular Genetics, Research Institute of Agricul- tural Resources, Ishikawa Agricultural College, Nonoichi-machi, Ishikawa 921-8836, Japan.