ARE YEAST CHITIN SYNTHASES REGULATED AT THE TRANSCRIPTIONAL OR THE POSTTRANSLATIONAL LEVEL

The three chitin synthases of Saccharomyces cerevisiae, Chs1, Chs2, and Chs3, participate in septum and cell wall formation of vegetative cells and in wall morphogenesis of conjugating cells and spores. Because df the differences in the nature and in the time of execution of their functions, the synthases must be specifically and individually regulated. The nature of that regulation has been investigated by measuring changes in the levels of the three synthases and of the messages of the three corresponding genes, CHS1, CHS2, and CAL1/CSD2/DIT101/KT12 (referred to below as CAL1/CSD2), during the budding and sexual cycles. By transferring cells carrying CHS2 under the control of a GAL1 promoter from galactose-containing medium to glucose-containing medium, transcription of CHS2 was shut off. This resulted in a rapid disappearance of Chs2, whereas the mRNA decayed much more slowly. Furthermore, Chs2 levels experienced pronounced oscillations during the budding cycle and were decreased in the sexual cycle, indicating that this enzyme is largely regulated by a process of synthesis and degradation. For CHS1 and CAL1/CSD2, however, a stop in transcription was followed by a slow decrease in the level of zymogen (Chs1) or an increase in the level of activity (Chs3), despite a rapid drop in message level in both cases. In synchronized cultures, Chs1 levels were constant during the cell cycle. Thus, for Chs1 and Chs3, posttranslational regulation, probably by activation of latent forms, appears to be predominant. Since Chs2, like Chs1, is found in the cell in the zymogenic form, a posttranslational activation step appears to be necessary for this synthase also.