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 complete DNA sequence of the yeast Saccharomyces cerevisiae chromosome XI has been determined. In addition to a compact arrangement of potential protein coding sequences, the 666,448-base-pair sequence has revealed general chromosome patterns; in particular, alternating regional variations in average base composition correlate with variations in local gene density along the chromosome. Significant discrepancies with the previously published genetic map demonstrate the need for using independent physical mapping criteria.

Saccharomyces cerevisiae strains with either three inactivated genes (triple disruptants) or four inactivated genes (quadruple disruptants) encoding the four acidic ribosomal phosphoproteins, YP1␣, YP1␤, YP2␣, and YP2␤, present in this species have been obtained. Ribosomes from the triple disruptants and, obviously, those from the quadruple strain do not have bound P proteins. All disrupted strains are viable; however, they show a cold-sensitive phenotype, growing very poorly at 23؇C. Cell extracts from the quadruple-disruptant strain are about 30% as active as the control in protein synthesis assays and are stimulated by the addition of free acidic P proteins. Strains lacking acidic proteins do not have a higher suppressor activity than the parental strains, and cell extracts derived from the quadruple disruptant do not show a higher degree of misreading, indicating that the absence of acidic proteins does not affect the accuracy of the ribosomes. However, the patterns of protein expressed in the cells as well as in the cell-free protein system are affected by the absence of P proteins from the particles; a wild-type pattern is restored upon addition of exogenous P proteins to the cell extract. In addition, strains carrying P-protein-deficient ribosomes are unable to sporulate but recover this capacity upon transformation with one of the missing genes. These results indicate that acidic proteins are not an absolute requirement for protein synthesis but regulate the activity of the 60S subunit, affecting the translation of certain mRNAs differently.The presence of a set of strongly acidic proteins is a universal feature of ribosomes from all organisms. These proteins have been involved in interactions with different supernatant factors during protein synthesis in prokaryotic as well as eukaryotic systems (see reference 16 for a review).In eukaryotic organisms, the acidic ribosomal proteins, called generically P proteins, are capable of being phosphorylated, and this modification drastically affects the interaction of the polypeptides with the ribosome in vitro (11,14). In fact, this process might be part of a control mechanism that, by regulating the amount of bound protein, can affect the activity of the particles in relation to the metabolic requirements of the cell (24).In Saccharomyces cerevisiae, four acidic ribosomal proteins have been reported and characterized; according to structural and functional considerations, they can be classified in two groups, YP1␣/␤ and YP2␣/␤, corresponding to the two types of acidic polypeptides present in mammalian ribosomes (35). Using gene disruption techniques, we previously found that the individual elimination of each polypeptide, and even the double disruption of any two of the four proteins, is not a lethal event for the cell, although the generation time is affected to different extents (20, 21). The effect is especially important when the genes encoding the two members of the same group are disrupted, since in this case the remaining acidic proteins present in the c...