Novel 3'-phosphoadenosine-5'-phosphatases from extremely halotolerant Hortaea werneckii reveal insight into molecular determinants of salt tolerance of black yeasts

The 3'-phosphoadenosine-5'-phosphatase encoded by HAL2 gene, is a ubiquitous enzyme required for the removal of the cytotoxic 3'-phosphoadenosine- 5'-phosphate produced during sulfur assimilation in eukaryotes. Salt toxicity in yeast and plants results from HAL2 inhibition by sodium or lithium ions. Two novel HAL2-like genes, HwHAL2A and HwHAL2B, have been cloned from saltern-inhabited extremely halotolerant black yeast Hortaea werneckii. Expression of both HwHAL2 isoforms was differentially inducible upon salt. When the HivHAL2 genes were transferred from such a halotolerant species into the salt sensitive Saccharomyces cerevisiae, the resulting organism can tolerate 1.8 M NaCl or 0.8 M LiCl, the highest reported salt concentrations at which S. cerevisiae can grow. With genetic and biochemical validation we demonstrated the critical HwHal2B sequence motif-the META sequence-common only to Dothideales fungi, with evident effect on the HwHal2B-dependent salt tolerance. These results may have significance for biosaline agriculture in coastal environments. (C) 2007 Elsevier Inc. All rights reserved.