AbstractA newly isolated gene,ESS1, was shown to encode a protein required for vegetative growth inSaccharomyces cerevisiae. The nucleotide sequence ofESS1revealed a 172 amino acid open reading frame predicting a highly basic, 19·5 kilodalton product. Although the gene was isolated by cross‐hybridization with the vertebrate v‐sisoncogene, the primary amino acid sequence bears only a slight resemblance to the p28sisprotein.ESS1was shown to be single copy in the yeast genome and transcriptionally active during logarithmic growth. It is located on the right arm of chromosome X, 6 centimorgans distal toilv3. The genetic map location indicates it is not allelic to any previously characterized mutation in this organism. Both inactivation ofESS1by gene disruption and overexpression by fusion to a heterologous promoter were detrimental to growth in both haploid and diploid cell types. Under non‐permissive conditions, the terminal phenotype of strains containing a suppressible amber mutation withinESS1was one of aberrant multibudded structures. Examination of this morphology indicates that loss ofESS1function may lead to a defect in cytokinesis or cell sepa
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