Sac3, an Snf1-like serine threonine kinase that positively and negatively regulates the responses of chlamydomonas to sulfur limitation

The Sac3 gene product of Chlamydomonas positively and negatively regulates the responses of the cell to sulfur limitation. In wild-type cells, arylsul fatase activity is detected only during sulfur limitation. The sac3 mutant expresses arylsulfatase activity even when grown in nutrient-replete medium , which suggests that the Sac3 protein has a negative effect on the inducti on of arylsulfatase activity. In contrast to its effect on arylsulfatase ac tivity, Sac3 positively regulates the high-affinity sulfate transport syste m-the sac3 mutant is unable to fully induce high-affinity sulfate transport during sulfur limitation. We have complemented the sac3 mutant and cloned a cDNA copy of the Sac3 gene. The deduced amino acid sequence of the Sac3 g ene product is similar to the catalytic domain of the yeast Snf1 family of serine/threonine kinases and is therefore classified as a Snf1-related kina se (SnRK). specifically, Sac3 falls within the SnRK2 subfamily of kinases f rom vascular plants. In addition to the 11 subdomains common to Snf1-like s erine/threonine kinases, Sac3 and the plant kinases have two additional sub domains and a highly acidic C-terminal region. The role of Sac3 in the sign al transduction system that regulates the responses of Chlamydomonas to sul fur limitation is discussed.