Iron dependent post-translational regulation of the bHLH transcription factor FIT in Arabidopsis thaliana

摘要

Iron (Fe) is an essential micronutrient for most organisms, but too high Fe contents can cause the formation of free radicals. Hence, Fe uptake must be tightly regulated. Root iron acquisition in non-graminaceous plants, like the model plant Arabidopsis thaliana, is achieved by reduction of soil Fe by the reductase FRO2 and its subsequent uptake by the metal transporter IRT1. The bHLH transcription factor FIT is required for high-level expression of FRO2 and IRT1 upon Fe deficiency. In this work we investigated post-transcriptional regulation of FIT. We found Fe dependent post-transcriptional regulation of FIT in way of constant turnover. Small amounts of active FIT were found to be sufficient to trigger the expression of FRO2 and IRT1. FIT protein stability relies on the presence of the signalling compound nitric oxide (NO). NO mediated stabilisation of FIT is independent of transcriptional regulation and is most probably achieved by counteracting proteasome dependent degradation of FIT. We summarise our results in an integrative model and based on this we made further efforts to identify post-translational modifications that could regulate FIT activity. Based on in silico prediction we identified four amino acids in the c-terminal part of FIT as putative phosphorylation sites. With newly generated FIT forms containing phosphomicking or non-phosphorylatable mutations we can draw further conclusions and suggest that phosphorylation may regulate FIT activity.