The incorporation and metabolic alteration of a variety of dietary fatty acids has been studied in wild‐type Saccharomyces cerevisiae and a fatty‐acid auxotroph which carries a genetic defect in its fatty‐acid synthetase complex. This mutant grows when supplemented with a singleven or odd chain‐length fatty acid ranging from 13–17 carbon atoms. Although incapable of fattyacid synthesis from acetate de novo, the mutant yeast still elongates the dietary fatty acids by one or more C2 units. The extent of elongation is dependent on the chain length of the fatty acid included in the growth medium. It is most evident with tridecanoic acid and insignificant with palmitic acid. All saturated fatty acids are desaturated to an appreciable but varying degree to the corresponding monounsaturated acids. No degradation to lower homologues is observed in the wild‐type, nor in the mutant cells. No evidence was obtained to indicate participation of the mutant fatty‐acid synthetase complex in the elongation process described. In wild‐type yeast, intracellular fatty acid biosynthesis is unimpaired by the addition of free fatty acids to the growth medium and apparently proceeds by a much faster rate than the uptake and elongation of fatty acids from the growth medium. A membrane‐bound enzyme analogous to acid : CoA ligase has been isolated from bakers' yeast and its chain‐length specificity studied. The optimal chain length with saturated fatty acids is 15 carbon atoms. Unsaturated fatty acids are generally better substrates. The role of fatty acid activation in determining the rate of fatty‐acid‐dependent yeastmutant growth is discussed.

I n Saccharomyces cerevisiae, the genetic linkage as well as the biochemical characteristics of nine different fatty-acid-synthetase complementation groups have been studied. By spore tetrad analysis, the alleles of these nine groups were established to be located on three genetpically unlinked DNA regions on the yeast genome. These regions consist of one separate gene and two clusters comprising two and three adjacent cistrons, respectively. In both clusters the alleles are closely linked within less than one map unit. The cistrons of the first cluster can be assigned to structural genes of the dehydratase and the second reductase, those of the second cluster to the first reductase and two proteins involved in the condensation reaction. Mutants of the separate fas-cistron produce a fully active fatty acid synthetase complex with all the component enzyme activities unimpaired under the assay conditions employed. The function of the gene product of this cistron is unknown, so far. From the pleiotropic fas-mutants, a fatty-acid-synthetase complex with residual component enzyme activities could not be isolated. It is concluded that due to the polar transcription of the clustered cistrons in these mutants either none or only an incomplete aggregate is formed with the concomitant inactivation of residual component enzymes.

By pairwise crosses, 75 different fatty acid requiring mutants of Saccharomyces cerevisiae were subjected to complementation analysis. From the data obtained, a linear complementation map was constructed comprising nine different complementation groups. Eight of these groups had a polar complementation pattern. They failed to complement not only with members of their own but also with those of one or several other complementation groups. In all crosses investigated, genetic complementation was paralleled by the restoration of fatty acid synthetase activity in the heterozygous diploid. Two component enzyme activities of the fatty acid synthetase complex were studied with representatives of five different complementation groups. Hereby, four different activity patterns have been observed. It is concluded that at least these complementation groups represent different cistrons of the fatty acid synthetase system, although intracistronic complementation cannot be excluded for other groups, so far. Qualitatively, the mutants of eight different complementation groups are susceptible to mutual, UV-induced, mitotic recombination. This finding, together with the pleiotropy observed in complementation and enzyme defects, suggests that several fas-genes are linked and coordinatedly transcribed.Zusammenfassung: Ein weiteres Beispiel für Genkopplung in Hefe: Das Fettsäure-Synthetase-System. 75 verschiedene fettsäurebedürftige Saccharomycescerevisiae-Mutanten wurden durch paarweise Kreuzung auf ihr Komplementationsverhalten untersucht. Die Mutanten bilden neun verschiedene Komplementationsgruppen und können in einer linearen Komplementationskarte dargestellt werden. Acht Gruppen haben polaren Charakter und komplementieren weder mit der eigenen noch mit einer oder mehreren anderen Gruppen. In allen untersuchten Heterozygoten war bei positiver Komplementation die Fettsäure-Synthetase im Extrakt wieder aktiv. Die Aktivitäten zweier Teilreaktionen des Komplexes wurden an Vertretern von fünf ver-schiedenen Komplementationsgruppen untersucht. Dabei wurden vier verschiedene Aktivitätsmuster beobachtet. Man kann annehmen, daß zumindest diese Gruppen verschiedene Cistrons der Fettsäure-Synthetase repräsentieren. Für die anderen Gruppen kann das Vorliegen von intracistronischer Komplementation noch nicht eindeutig ausgeschlossen werden. Die Mutanten von acht Komplementationsgruppen können durch UV-Bestrahlung zu gegenseitiger mitotischer Rekombination angeregt werden. Hieraus und aus der beobachteten Pleiotropie im Komplementations-und Enzymmuster verschiedener Gruppen wird geschlossen, daß zumindest einige der fas-Gene in Hefe gekoppelt vorliegen und koordiniert abgelesen werden.

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