Partial isolation of two Scenedesmus acutus genes homologous to C. reinhardtii ATP sulfurylase

In the freshwater green alga Scenedesmus acutus, sulfur starvation induces a transient increase of Cr(VI) tolerance, both in the wild type and in a Cr-tolerant strain, associated to an increased capacity of sulfur uptake and assimilation leading to an increase in cysteine synthesis during the recovery in standard medium (Gorbi et al 2007). Despite being chromium tolerance not related to the induction of phytochelatins, the tolerance to this metal seems strictly connected to sulfur metabolism (Pereira et al., 2008; Schiavon et al., 2008). It is well known that sulfur starvation induces a huge variation in gene expression in Chlamydomonas reinhardtii (Zhang et al 2004) as well as in other algae and in land plants (Bochenek et al., 2013). Among the genes overexpressed there are plasma membrane high affinity sulfate transporters and many enzymes of the sulfate assimilation pathway. Once sulfate entered the cell, it is activated by the enzyme ATP sulfurylase which catalyze the first reaction of the reductive/assimilation pathway. In order to verify if differences in ATP sulfurylase expression is involved in the observed transient increase in Cr-tolerance, as well in the differential sensitivity shown by the two S. acutus strains, we tried to clone fragments of ATP sulfurylase genes. Primers designed on the sequence of C.reinhardtii and used both on genomic DNA and cDNA from 24h S-starved algae allowed the cloning of two isoforms homologous to C. rehinardtii ATS1 and ATS2. The cloned fragments share high sequence homology but differ for some specific aminoacid residues nearby the active site suggesting a different substrate affinity of the two isoforms. Moreover preliminary results show a different expression of the two enzymes, being ATS1 likely expressed in a constitutive manner while ATS2 is induced by sulfur starvation. Gorbi et al., 2007, Aquat. Toxicol. 84:457-464 Bochenek et al., 2013, New Phytol. 199, 650-662 Pereira et al., 2008, Toxicol Sci 106(2): 400-412 Schiavon et al., 2008, J. Environ. Qual. 37: 1536-1545 Zhang et al. 2004 Eukariotic Cell Vol.3 , No.5: 1331-1348