Protoplasmic incompatibility and the genetic control of stable messenger RNAs.

In Podospora anserina, protoplasmic incompatibility due to the combination of nonallelic genes is suppressed by the addition of a recessive mutation in the modA gene to a dominant mutation in the modB gene. The effect of the modA and modB mutations on several features of protoplasmic incompatibility was investigated. Results show that the modA mutation suppresses two of them: the shutoff of RNA synthesis and the inhibition of synthesis of normal proteins. The suppression of a third feature of protoplasmic incompatibility, i.e., the synthesis of specific proteins, involves both the modA and the modB mutations. Employing the dihydrostreptomycin sensitivity of the modA gene product, it is shown that the modA gene controls the synthesis of one of these proteins (laccase III) at a posttranscriptional level. Concerning the modB gene, its effect was studied by means of a thermosensitive recessive mutation in this gene. It is demonstrated that the presence of this mutation results in the occurrence, in the cell extract, of enzyme activities (laccase III and its associated activities) and polypeptides specific to protoplasmic incompatibility. Furthermore, it could be deduced that the modB gene operates on these syntheses at a posttranscriptional level.