The Effect of Cycloheximide, Cycloheximide Analogues and Azaserine on Asparagine Synthesis in Corn Root-Tips

Abstract

The experiments in this thesis were undertaken to: 1) compare the effects of cycloheximide and azaserine on asparagine synthesis in root-tip sections of corn; 2) study the effect of protein synthesis on asparagine synthesis in root-tip sections by the use of analogues of cycloheximide; and 3) study the properties of asparagine synthetase extracted from corn roots.When [2-1 4c]-acetate is fed to excised root-tip sections of corn pre-incubated in the presence of cycloheximide, protein synthesis is inhibited. The effect is almost immediate. Within the amide fraction, the levels of glutamine formed in these sections rises over the 3 hour pre-incubation period. Asparagine synthesis gradually declines over the same period. In similar experiments performed with azaserine in the pre-incubation media, protein synthesis was not markedly inhibited. Glutamine levels were immediately increased over the 3 hour period. The effect on asparagine synthesis was also rapid. In contrast to the situation with cycloheximide, the effect of azaserine on amide synthesis is constant over a 3 hour period. Two analogues of cycloheximide - cycloheximide acetate and streptovitacin A - were found to produce effects similar to that of cycloheximide. These analogues were found to inhibit both protein synthesis and asparagine synthesis after a 3 hour exposure period. Six other analogues did not show marked inhibitory effects on either protein synthesis or asparagine synthesis. Asparaqine synthetase activity was found in extracts from corn seedling tissues.. However, assays for asparagine synthetase revealed that the activity was low and 'that other aspartate utilizing enzymes were probably active in the extracts. From the results of this investigation and those of earlier published results a model has been proposed in order to explain the regulation of asparagine synthesis in corn roots.