DhARO4 induction and tyrosine nitration in response to reactive radicals generated by salt stress in Debaryomyces hansenii.

It has been previously reported that growth of Debaryomyces hansenii in 2 M NaCl induced the expression of ARO4. This gene codifies for DhAro4p, involved in the synthesis of the amino acid tyrosine. In this work we studied the activity of DhAro4p upon salt stress; a higher activity was observed in cells grown with 2 M NaCl, but tyrosine levels were not increased. On the other hand, the addition of tyrosine to the saline medium significantly enhanced the growth of D. hansenii. It was found that the oxidized form of tyrosine, 3-nitrotyrosine, increased in the presence of salt. Since NaCl protects against oxidative stress in D. hansenii (Navarrete et al., 2009), we propose that a protective pathway is the de novo synthesis of tyrosine and its immediate oxidation to 3-nitrotyrosine to counteract oxidative stress generated by salt stress, so we measured the production of reactive oxygen species (ROS) and nitric oxide (NO⁻) in D. hansenii after growing in 2 M NaCl. Results showed the presence of NO⁻ and the increased production of ROS; this is probably due to an increased respiratory activity in the cells grown in the presence of salt. Our results demonstrate that upon salt stress D hansenii responds to oxidative stress via the transcriptional activation of specific genes such as DhARO4.