Using a Saccharomyces cerevisiae strain having the activities of serine O-acetyl-transferase (SATase), O-acetylserine/ O-acetylhomoserine sulphydrylase (OAS/OAH SHLase), cystathionine-synthase (-CTSase) and cystathionine-lyase (-CTLase), we individually disrupted CYS3 (coding for-CTLase) and CYS4 (coding for-CTSase). The obtained gene disruptants were cysteine-dependent and incorporated the radioactivity of 35 S-sulphate into homocysteine but not into cysteine or glutathione. We concluded, therefore, that SATase and OAS/OAH SHLase do not constitute a cysteine biosynthetic pathway and that cysteine is synthesized exclusively through the pathway constituted with-CTSase and-CTLase; note that OAS/OAH SHLase supplies homocysteine to this pathway by acting as OAH SHLase. From further investigation upon the cys3-disruptant, we obtained results consistent with our earlier suggestion that cysteine and OAS play central roles in the regulation of sulphate assimilation. In addition, we found that sulphate transport activity was not induced at all in the cys4-disruptant, suggesting that CYS4 plays a role in the regulation of sulphate assimilation.

Summary The phytohormone ABA and the transcription factor ABSCISIC ACID INSENSITIVE 3 (ABI3)/VIVIPAROUS 1 (VP1) function in protecting embryos during the desiccation stage of seed development. In a similar signaling pathway, vegetative tissue of the moss Physcomitrella patens survives desiccation by activating downstream genes (e.g. LEA1) in response to ABA and ABI3. We show that the PpLEA1 promoter responds to PpABI3 primarily through the ACTT‐core element (5′‐TCCACTTGTC‐3′), while the ACGT‐core ABA‐responsive element (ABRE) appears to respond to ABA alone. We also found by yeast‐two‐hybrid screening that PpABI3A interacts with PpNF‐YC1, a subunit of CCAAT box binding factor (CBF)/nuclear factor Y (NF‐Y). PpNF‐YC1 increased the activation of the PpLEA1 promoter when incubated with PpABI3A, as did NF‐YB, NF‐YC, and ABI3 from Arabidopsis. This new response element (ACTT) is responsible for activating the ABI3‐dependent ABA response pathway cooperatively with the nuclear factor Y (NF‐Y) complex. These results further define the regulatory interactions at the transcriptional level for the expression of this network of genes required for drought/desiccation tolerance. This gene regulatory set is in large part conserved between vegetative tissue of bryophytes and seeds of angiosperms and will shed light on the evolution of this pathway in the green plant lineage.

Using a Saccharomyces cerevisiae strain having the activities of serine O‐acetyl‐transferase (SATase), O‐acetylserine/O‐acetylhomoserine sulphydrylase (OAS/OAH SHLase), cystathionine β‐synthase (β‐CTSase) and cystathionine γ‐lyase (γ‐CTLase), we individually disrupted CYS3(coding for γ‐CTLase) and CYS4 (coding for β‐CTSase). The obtained gene disruptants were cysteine‐dependent and incorporated the radioactivity of 35S‐sulphate into homocysteine but not into cysteine or glutathione. We concluded, therefore, that SATase and OAS/OAH SHLase do not constitute a cysteine biosynthetic pathway and that cysteine is synthesized exclusively through the pathway constituted with β‐CTSase and γ‐CTLase; note that OAS/OAH SHLase supplies homocysteine to this pathway by acting as OAH SHLase. From further investigation upon the cys3‐disruptant, we obtained results consistent with our earlier suggestion that cysteine and OAS play central roles in the regulation of sulphate assimilation. In addition, we found that sulphate transport activity was not induced at all in the cys4‐disruptant, suggesting that CYS4 plays a role in the regulation of sulphate assimilation. Copyright © 1999 John Wiley & Sons, Ltd.

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We examined how the activity of O‐acetylserine and O‐acetylhomoserine sulphydrylase (OAS/OAH) SHLase of Saccharomyces cerevisiae is affected by sulphur source added to the growth medium and genetic background of the strain. In a wild‐type strain, the activity was repressed if methionine, cysteine or glutathione was added to the growth medium. However, in a strain deficient of cystathionine γ‐lyase, cysteine and glutathione were repressive, but methionine was not. In strains deficient of serine O‐acetyltransferase (SATase), OAS/OAH SHLase activity was low regardless of sulphur source and was further lowered by cysteine and glutathione, but not by methionine. From these observations, we concluded that S‐adenosylmethionine should be excluded from being the effector for regulation of OAS/OAH SHLase. Instead, we suspected that S. cerevisiae would have the same regulatory system as Escherichia coli for sulphate assimilation; i.e. cysteine inhibits SATase to lower the cellular concentration of OAS which is required for induction of the sulphate assimilation enzymes including OAS/OAH SHLase. Subsequently, we obtained data supporting this speculation.