Abstract
The spontaneous reversion frequency of homocysteine requiring strain (4-24) of Ustilago maydis was determined as 23×10-5 per sporidium on the minimal media supplemented with 1×10-5mg/ml methionine. This genetic block in the biochemical pathway between cystathionine and homocysteine has been elucidated as due to one genic step. In contrast to the current opininon, the reversion in the present strain, now observed, does not take place in one step but step by step through two distinct transient steps. It reverted in sequence of 4-24→PRT 1→PRT 2→CRT; where PRT and CRT denoted respectively, partial reversion type and complete reversion type. It is noted that the two transient types, PRT 1and PRT 1, are as stable as the original strain.
Studies of the growth rate of these reversion types have shown that the present partial reversion is explicable by the leakage, the mechanism of which, however, is not yet settled in the present experiments.
