Deletion of HOG1 led to de-repression of MCFOs and to increased ferric reductase activity. (A) SDS-PAGE analysis of MCFOs extracted from the WT (SC5314), the reference strain (DAY286), Δhog1 (JMR114) and Δpbs2 (JJH31) mutants grown in YPD at 30°C for 16 h. For the whole gel see Additional file 2. (B) Cell surface ferric reductase activity of SC5314 (WT), DAY286 (reference strain) and Δhog1 (JMR114) under both restricted iron (RIM) and sufficient iron (YPD) conditions. Mean values and standard deviations of three independent experiments (n = 3) are shown. *** denotes P < 0.001 (student’s t-test). The ferric reductase of activity of the WT strain (SC5314) grown in YPD was set as 100%. (C) SDS-PAGE analysis of MCFOs extracted from Δhog1 (JMR114) grown in sufficient iron (YPD) or restricted iron (RIM) medium at 30°C for 3 h. Identity of the MCFOs was confirmed by mass spectrometry. For the whole gel see Additional file 3.