Although a great deal is known about the chemical structure and enzymatic mechanisms involved in the assembly of yeast mannoproteins, little is known about the regulation of mannan biosynthesis. The aim of this study, therefore, was to investigate the control mechanisms involved. Two approaches were used. Firstly, the potential role of low molecular-weight effectors of mannosyltransferases was investigated. Secondly, the possible existence of an activation-inactivation mechanism of control, analogous to that which operates during chitin synthesis was explored. Mannosyltransferase activity in sphaeroplast lysates and washed membranes prepared from Saccharomyces cerevisiae X2180 was measured by following the incorporation of [14C]cjmannose from GDP-[14C]nBnnose into material precipitable with cold 0.3M perchloric acid. Mannosyl- transferase activity was optimal at low enzyme protein concentrations (l mg/ml), 5
