The recombinant coat protein (CP) of Sesbania mosaic virus (SeMV; genus Sobemovirus) was found to self-assemble into capsids encapsidating 23S rRNA and CP mRNA in Escherichia coli. The CP lacking 22 amino acids from the N-terminus assembled into stable T = 3 capsids that appeared similar to SeMV, indicating that the N-terminal 22 amino acid residues are dispensable for T = 3 assembly. Two distinct capsids, T = 1 and pseudo T = 2, were observed when the N-terminal 36 amino acids encompassing the arginine-rich motif (N-ARM) were removed. Only T = 1 particles were observed upon deletion of 65 amino acids from the N-terminus, which also included the sequence element for the beta-annulus. These results reveal that N-ARM acts as a molecular switch in regulating T = 3 assembly. Formation of stable pseudo T = 2 particles shows that pentamers of AB dimers could nucleate assembly at icosahedral-5-folds. Capsids assembled from the N-terminally truncated proteins also encapsidated 23S rRNA and CP mRNA, suggesting the presence of sites outside the N-terminal 65 residues that may be involved in RNA--protein interactions.