STRUCTURAL COMPARISON OF THE PLANT SATELLITE VIRUSES

Detailed structures are now available for three plant satellite viruse s, satellite tobacco necrosis virus (STNV), satellite tobacco mosaic v irus (STMV), and satellite panicum mosaic virus (SPMV). It is, therefo re, possible to compare the tertiary structure of viral protein subuni ts, their quaternary interactions, and the interactions of protein sub units with the RNA genome. This analysis indicates that, in spite of c ommon function and preservation of a ''jelly-roll'' motif in the prote in monomer, the three viruses are remarkably different The differences include the arrangement of secondary structural elements, interaction s of adjacent subunits, and the disposition of subunits relative to ic osahedral symmetry axes. In each of the three viruses, however, the na rrow end of the jelly roll forms fivefold contacts. The fivefold prote in interactions are organized about a Ca2+ ion for STNV, an anion for STMV, and, apparently, neither of these for SPMV. Low-resolution neutr on diffraction studies using H2O/D2O solvent contrast variation reveal ed the general location of the RNA genome within the STNV. In the case of SPMV, regions of electron density on the interior of the capsid co uld be assigned to RNA, although it was not possible to model the nucl eic acid. Only for STMV was nucleic acid visible in election density m aps, and this was manifested as double-helical RNA segments associated with each coat protein dimer. The observations presented here provide no support for any common evolutionary relationship. (C) 1995 Academi c Press, Inc.