Among 52 Saccharomyces cerevisiae fatty acid synthetase (fas)mutants screened for their ability to incorporate 14C‐labeled pantothenic acid into the fatty acid synthetase multienzyme complex, especially those of fas‐complementation group VII had lost this ability. The purified fatty acid synthetase complexes of all the 19 fas‐mutants available from this group were shown to be free of 14C‐labeled pantothenic acid. The amount of pantothenate incorporated into the enzymes of several other fas‐mutants of the non‐pleiotropic complementation groups II, V, VI and VIII, however, was similar to that observed with wild‐type fatty acid synthetase. The purified pantothenate‐free fatty acid synthetases of five different group VII fas‐mutants have been tested for the seven known component enzymes of the complex. In all mutants, only the β‐ketoacyl synthetase was inactive whereas in vitro all the other partial activities were unimpaired. By sodium dodecylsulfate‐polyacrylamide‐gel electrophoresis, no differences could be observed between the protein structure of the pantothenate‐free mutant fatty acid synthetase and that of the wild‐type comp ex. Both were separated into three different components A, B and C with molecular weights of 185000, 180000 and 177000, respectively. However, some fas‐mutants consist only of the components A and B, others only of B and C. The study of various [14C]‐pantothenate‐labeled mutant fatty acid synthetases suggests that component C originates from A presumably by limited proteolysis. In the undegraded complex AB, [14C]pantothenate is only associated with the component A. Since in one of the mutants studied A is completely converted to C, it is concluded that A is one distinct component rather than a group of components with identical molecular weights. It is tentatively suggested that the gene product of fas 2, one of the two known fatty‐acid‐synthetase gene clusters in yeast, is only one, single and multifunctional polypeptide chain. Therefore, it appears that in yeast, the acyl carrier protein is not an individual protein component of the fatty acid synthetase complex, but only a distinct region of the multifunctional polypeptide chain encoded by fas 2.

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The main property of an Amt- (ammonium transport negative) mutant of Klebsiella pneumoniae is its inability to accumulate NH4+ intracellularly. When growing on nitrogen sources other than NH4+, the mutant constantly looses NH3 by diffusion. This loss results in poor growth. The NH3 excretion suggests the existence of a futile cycle (NH3 loss/NH4+ reabsorption) in the wild type and possibly other bacterial strains, which do not constantly excrete NH3.

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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.