Different alterations of the ribosomal protein S7 lead to opposite effects on translational fidelity in the fungus Podospora anserina

In the fungus Podospora anserina, the su12-1 mutation was previously found to decrease translational accuracy and to alter the ribosomal protein S7. The mutant protein is more basic than the wild type. Among the revertants of the two ribosomal mutations su12-1 and su12-2, 29 contained a second mutation very closely linked to su12. Biochemical analysis of these revertants by functional poly(U) tests and electrophoretical study of the ribosomes led to two conclusions. First, some revertant strains contain new mutant forms of S7. This suggests that su12 is the structural gene for the ribosomal protein S7. Second, the su12-2 revertants display antisuppressor properties in vivo and in vitro (i.e. increased translational accuracy). The electrophoretical patterns of their ribosomal proteins show new, more acidic, forms of S7. Therefore, su12 can be mutated towards either a lower or a greater translational accuracy corresponding to two opposite modifications of the global charge of the ribosomal protein S7. A more acidic form than wild type leads to increased accuracy and a more basic form to decreased accuracy.