Analyse der Sorbose-Resistenz von Neurospora crassa an Doppelmutanten; ein Beitrag zur Genetik des aktiven Transports. II

Summary

4 sorbose-resistant Neurospora crassa mutants, whose resistance was found to be due to a decreased rate of sorbose uptake, were crossed in two pairs with each other. In both crosses mutants were combined which contained mutational changes at sites not closely linked with each other, and therefore representing separate genes. Among the ascosporeisolates, double mutant recombinants were selected. This was facilitated by suitable markers in one of the crosses.

Growth of these double mutants with and without added sorbose was compaired with that of a number of parental- and wildtype-reisolates from the same crosses. In spite of the fact that the double mutant sor ^r A-10/sor ^r C-17 without sorbose grows more slowly than the parental- and wildtype-reisolates, it proved much more resistant than these in the presence of sorbose. The same was true for the second double mutant sor ^r A-2/sor ^r-6.

Since it can be shown that probably none of the two A-mutants used (sor ^r A-10 and sor ^r A-2) forms any active A-protein, it seems unlikely that the respective pairs of genes in the wildtype specify proteins catalyzing a “reaction sequence”. Another possibility envisages a “compound-permease” made up of subunits specified by the respective genes, but is not very likely either. A third possibility, which explains satisfactorily all earlier as well as the present results, is that the resp. genes code for identical or different proteins which as “isopermeases” catalyze one and the same reaction step in the course of the uptake of sorbose.