Sulfur metabolism and chromium tolerance in Scenedesmus acutus: the role of OAS-TL and Cysteine Desulfhydrase activities.

In plant and algal cells, the sulfur metabolic pathway is critical for mitigating damage induced by heavy metals, a process known as sulfur-enhanced defense (SED). Previous research conducted on two strains of Scenedesmus acutus, wild type (wt) and chromium-tolerant strain (Cr-t), evidenced a transient tolerance increase in both strains after a period of sulfur starvation, related to the enhancement of sulfur uptake following nutrient re-supply and leading to an increase in cysteine content, significantly higher in the Cr-t strain [1]. At the core of sulfur metabolism is the cysteine synthase complex (CSC), composed by a hexamer of Serine Acetyltransferase (SAT) and two dimers O-acetyl-Serine (Thiol) Lyase (OAS-TL) [2], present in different cell compartments with different roles in cysteine homeostasis. While SAT in association with OAS-TL synthesizes OAS, the free OAS-TL dimer catalyzes cysteine synthesis from O-acetyl-serine (OAS) and sulfide produced by Sulfite Reductase (SIR). In S. acutus, we identified one SIR, two SAT and four OAS-TL isoforms showing different basal expression levels and inducibility by sulfur starvation being SaSAT1, SaOAS-TL4, and, notably, SaOAS-TL2 the most inducible forms. Enzymatic assays confirmed increased OAS-TL activity following sulfur deprivation in both strains and a decrease after nutrient re-supply. SaOAS-TL4, phylogenetically closer to terrestrial plants, exhibited the highest basal expression and strongest induction under sulfur starvation, while SaOAS-TL2 was not only strongly induced, but it was also the only OAS-TL isoform not expressed constitutively or expressed at near-undetectable levels. Phylogenetic analysis confirmed that SaOAS-TL2 is the most structurally divergent isoform, lacking the SAT-binding site and exhibiting differences in the active site responsible for sulfide interaction. These characteristics suggest its potential role as a free dimer or as an L-cysteine desulfhydrase supporting the recycle of organic sulfur during starvation. Its presence in non-photosynthetic organisms supports the hypothesis that this may be the cytosolic or mitochondrial isoform. Extra plastidic sulfide production can indeed be substained by OAS-TLs desulfydrase side activity [3] and from the activity of specific DES enzymes which can contribute to recycling organic sulfur. The analysis of L- and D-cysteine desulfhydrase activities revealed an increase following seven days of sulfur deprivation in both strains. Across all tested conditions, D-cysteine desulfhydrase activity was consistently higher than that of L-cysteine desulfhydrase, with the difference being more pronounced in the Cr-tolerant strain, especially under sulfur starvation. Notably, the expression level of the gene encoding D-cysteine desulfhydrase was significantly higher in the Cr-t strain than in the wt, suggesting a greater presence of D-cysteine in the Cr-t strain as a possible strategy of organic sulfur accumulation to be mobilized in response to stress.