The yeast gene ALG1 encodes a mannosyltransferase which participates in the formation of the lipid-linked precursor oligosaccharide for N-glycosylation by catalyzing the formation of dolichol pyrophosphate (Dol-PP)-GlcNAc2Man from GDP-Man and Dol-PP-Glc-NAc2. The DNA region including the ALG1 gene was sequenced and found to contain an open reading frame of 1347 bases. The predicted ALG1 protein contained 449 amino acids (51.9 kDa) with a hydrophobic region near the amino terminus, suggesting it was an integral membrane protein, and four potential sites for N-glycosylation. Disruption of the ALG1 open reading frame by insertion of the URA3 gene showed that the ALG1 gene was essential for viability. Northern analysis and transcript protection showed that the ALG1 gene was transcribed into two classes of messages which differed by about 100 bases at their 5' end and shared a common 3' end. In actively growing cells the short transcript predominated, but as growth slowed the long transcript was more prevalent. The short transcript was predicted to encode the ALG1 protein while the long transcript was predicted to encode a 74-amino acid protein of unknown function. Disruption of the 74-amino acid reading frame, which ended 42 bases upstream of the potential start codon for the ALG1 protein, showed it was not essential for viability.