Regulation of the hexosamine biosynthetic pathway in the water mold Blastocladiella emersonii: Sensitivity to endproduct inhibition is dependent upon the life cycle phase

Abstract

Chitin, a homopolymer of N-acetylglucosamine (GlcNAc), is the major macromolecular constituent of Blastocladiella emersonii cell walls. Zoospores do not possess a wall nor do they contain sufficient total hexosamine to account for the chitin content of the wall abruptly formed during germination. UDPGlcNAc, both the endproduct of hexosamine biosynthesis and the substrate for chitin synthesis, is present in zoospores in sufficient concentration to inhibit the first hexosamine pathway-specific enzyme activity. Net chitin accumulates in register with dry weight during exponential growth, but does not accumulate appreciably during the succeeding sporulation phase. Predicted relationships among net rates of chitin synthesis, UDPGlcNAc concentrations, and UDP plus UTP concentrations throughout the life cycle are explored, as are the assumptions upon which the predictions were based. We find that the sensitivity of the first hexosamine pathway-specific enzyme to endproduct inhibition is not constant throughout the life cycle; sensitivity is very high in the zoospore phase, decreases dramatically during germination, remains very low through the growth phase, and increases gradually to the zoospore level during sporulation. The organism appears to have evolved endproduct regulation in this case as an adaptation to "hard-times" phases of the life cycle-i.e., as a safeguard against overproduction of end product (UDPGlcNAc) when its utilization in cell wall (specifically chitin) synthesis is curtailed. Conversely, the organism effectively discards this mode of regulation during "good times," when the demands for end product are evidently greater than endproduct inhibition would otherwise permit.