In fungi and yeasts, the conversion of acetate to acetyl-CoA is catalyzed by the enzyme acetyl-CoA synthetase (ACS). A single ACS form has been reported for some filamentous fungi, and the duplication of the structural ACS gene, producing ACS1 and ACS2 proteins, has been reported in yeasts. In S. cerevisiae, the ACS1 gene is induced by acetate and repressed by glucose, meanwhile the ACS2 gene encodes a protein with a lower affinity for acetate and is constitutively expressed. ACS also catalyses the synthesis of adenosine polyphosphates during yeast sporulation, a process which is induced by acetate. ACS2 will provide cytosolic acetyl-CoA for biosynthetic purposes in a pyruvate dehydrogenase bypass. In Kluyveromyces lactis, the KlACS1 and KlACS2 genes show similar transpcriptional regulation to their homologues in S. cerevisiae, meanwhile in Zygosaccharomyces bailii it has been cloned a gene, coding for acetyl-CoA synthetase, that shares significant homology with the products of ACS2 genes from S. cerevisiae and K. lactis. On the other hand, in the filamentous fungus Phycomyces blakesleeanus, we have purified and characterized two forms of acetyl-CoA synthetase (ACS1 and ACS2). ACS1, encoded by the gene facA, was induced by acetate and repressed by glucose at the transcriptional level. ACS2, not encoded by the gene facA, was detected as a response to carbon starvation both in the wild type and in a facA- mutant. This isoform is a stress protein, expressed in an adverse environment for the fungus, and it could participate in a pyruvate dehydrogenase bypass.
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