The entire DNA sequence of chromosome III of the yeast Saccharomyces cerevisiae has been determined. This is the first complete sequence analysis of an entire chromosome from any organism. The 315-kilobase sequence reveals 182 open reading frames for proteins longer than 100 amino acids, of which 37 correspond to known genes and 29 more show some similarity to sequences in databases. Of 55 new open reading frames analysed by gene disruption, three are essential genes; of 42 non-essential genes that were tested, 14 show some discernible effect on phenotype and the remaining 28 have no overt function.

The YDp plasmids (Yeast Disruption plasmids) are pUC9 vectors bearing a set of yeast gene disruption cassettes, all uniform in structure and differing only in the selectable marker used (HIS3, LEU2, LYS2, TRP1 or URA3). The markers, surrounded by translational termination codons, are embedded in the slightly modified sequence of the pUC9 multiple cloning sites.

Evidence is presented, based on sequence comparisons and secondary structure prediction, of the presence of a DNA-binding and dimerization helix-loop-helix motif in the yeast transcription activator PHO4. Interest in the existence of this first known motif in yeast is discussed.

The yeast PHO regulatory genes are indispensable for the transcriptional control of structural genes involved in phosphate metabolism. Some of these regulatory genes have pleiotropic functions apparently independent of phosphate metabolism. Our results point to an involvement in the life cycle of the PHO2 activator and the PHO85-negative factor and to a reduced ability of the negative pho regulatory mutants, pho80 and pho85, to grow on non-fermentable carbon sources.

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