Altered gene expression in Aspergillus nidulans in response to salt stress

Aspergillus nidulans was adapted to grow in medium amended with up to 2 M NaCl (osmotic potential, -11 MPa). Changes in gene expression associated with protracted exposure to NaCl (salt-adaptation) and transient exposure to NaCl (salt-shock) were analyzed at the level of protein and mRNA pools. Salt stress responses involved both reduction or down-regulation and increase or up-regulation in levels of certain proteins as well as de novo synthesis of other proteins. Two dimensional electrophoretic analysis revealed six proteins in the unadapted culture that were not detected in either salt-adapted or salt-shocked cells. Forty-one novel proteins and eight up-regulated proteins were detected in the salt-adapted culture, whereas seventeen de novo proteins and nine up-regulated proteins were detected in the salt-shock culture. Only two of the de novo proteins were shared by salt-adapted and salt-shocked cells. Comparative analysis of in vitro translation products during salt-adaptation and salt-shock suggests an increase in the steady-state level of several transcripts. Increase in the ropy number of ribosomal RNA genes was also apparent under salt stress. These alterations indicate that salt-adaptation and salt-shock differ, although both involve complex changes in gene expression.