Biochemical and genetic studies on galactosamine metabolism in Neurospora crassa.

In Neurospora, galactosamine can be released from the cell wall and from an alcohol-soluble compound by acid hydrolysis. All of the detectable alcohol-soluble galactosamine was present as uridine diphospho-2-acetamido-2-deoxy-D-galactose (UDPGalNAc). The results of pulse-labeling studies and enzymatic assays indicated that UDPGalNAc was synthesized via the epimerization of uridine diphospho-2-acetamido-2-de+xy-D-glucose (UDPGlcNAc). A single-gene morphological mutant, doily (do), which grew at less than 4% the rate of the wild-type strain, had 3% of the wild-type UDPGalNAc content and 0.5% of the wild-type level of cell wall galactosamine but normal levels of UDPGlcNAc and cell wall glucosamine. Cell extracts of the doily cultures containing only 20% of the specific activity of UDPGlcNAc-4-epimerase found in the extracts of wild-type cultures. Two types of faster-growing partial revertants of the doily strain were isolated. One type had an intermediate level of both alcohol-soluble and cell wall galactosamine. A second type had an intermediate level of alcohol-soluble galactosamine but low levels of cell wass galactosamine. Genetic analyses indicated that the reverse mutations had occurred at the do locus in both types. This finding that cell wall glucosamine synthesis and growth rate can be separated genetically indicates that mutations at the do lucus lead to pleiotropic effects.