Temperature-sensitive mutants of the yeast fatty-acid-synthetase complex

Abstract

By genetic complementation analysis, 88 independently isolated temperature-sensitive fatty acid synthetase mutants have been assigned to the six different fas-complementation groups II (fas 1), III (fas 1), Vb (fas 1), VI (fas 2), VIII (fas 2) and IX (fas 2). The complementation groups Va, Vc, Vd, IV and VII observed among nonconditional fas-mutants have not been found among the temperature-sensitive strains studied. From the failure to detect pantetheine-deficient conditional fas-mutants it is concluded that the yease acyl-carrier protein has an exceptionally stable tertiary structure. Furthermore, the lack of temperature-sensitive mutants of complementation group IV possibly indicates that this group specifically represents only nonsense and frameshift mutations. Almost half of the temperature-sensitive fas 1 and fas 2 mutants studied exhigited non-complementing characteristics. These results confirm the existence of non-complementing fas1 and fas2 missense mutations. From this it is concluded that both fatty acid synthetase loci encode multifunctional polypeptide chains rather than several monofunctional component enzymes. The possible existence of an independent acyl-carrier protein, as suggested by the genetic data reported in this study, is discussed. With 10 different temperature-sensitive fas1 and fas2 mutants the dependence of cellular growth rates on growth temperature and fatty acid supplementation was determined. With all mutants studied fatty-acid-independent growth was completely suppressed at non-permissive temperatures (34 -37 degrees C). In fatty-acid-supplemented media, however, these mutants exhibited the same growth characteristics as wild-type yeast cells. In contrast to this, wild-type yeast growth was found to be fatty-acid-independent at all temperatures studied. Other than in vivo, the purified fatty acid synthetase isolated from five different temperature-sensitive fas1 and fas2 mutants exhibited in vitro no increased thermolability compared to the wild-type enzyme. From this it is concluded that the specific conformation of fatty acid synthetase subunits either forms only at the ribosomal level during translation, or that this conformation is stabilized by the assembly of subunits into the multienzyme complex structure.