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.

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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.

Iron uptake in Saccharomyces cerevisiae involves at least two steps: reduction of ferric to ferrous ions extracellularly and transport of the reduced ions through the Iron is an indispensable element for living organisms. Oxygen storage and transport in plants and animals (leghemoglobins, hemoglobin, myoglobin, and hemerythrin), respiration, photosynthesis and electron transport (cytochromes), and nucleic acid metabolism (ribonucleotide reductase) are some examples of biological functions carried out principally by iron-containing proteins.Two features render iron an arduous metal to be handled by living organisms. The first is its availability; although it is the second most abundant metal in the earth's crust, iron is found primarily in the ferric (Fe3") state, forming hydroxides or salts of very low solubility and thus biologically inaccessible by simple mechanisms. The second is its toxicity; iron, in conjunction with oxygen, is a generator of hydroxyl radicals, which have a variety of toxic effects on cells.Living organisms as diverse as prokaryotes and mammals have developed a variety of mechanisms to overcome the problem of iron bioavailability as well as to regulate the iron concentration in biological fluids and cell compartments, in order to ensure its proper function as opposed to its toxic effects (reviewed in references 7 and 16). Two principal mechanisms have been described for iron uptake. One involves the use of iron chelators, either small molecules such as the siderophores secreted by bacteria, fungi, and plants or polypeptides such as transferrin and lactoferrin which are found in biological fluids of higher eukaryotes. All of these molecules keep ferric ions in a soluble form and deliver them to the cells mainly by receptor-mediated endocytosis. The second mechanism for iron uptake involves an initial reduction step in the external vicinity of the plasma membrane followed by internalization of the ferrous ions by means of ion transporters.Membrane-associated ferric reductase activity has been de-* Corresponding author. Mailing address: