The Plant Mediator Complex in the Transcriptional Response to Low Temperature.

Abstract

SENSITIVE TO FREEZING6 (SFR6) is the MEDIATOR16 (MED16) subunit of the plant Mediator complex and is shown here to be required for the recruitment of Pol II and the Mediator complex to cold-inducible C-repeat binding factor (CBF)-controlled genes. In addition, the MED2 and MED14 subunits are also required for Pol II recruitment to these genes. Mutant lines impaired in expression of SFR6/MED16, MED2 and MED14 subunits showed impaired expression of many, but not all cold-inducible genes. Some cold-inducible genes that do not contain C-repeat element (CRT) motifs in their promoters were also misregulated in sfr6/med16, mediator2 (med2) and mediator14 (med14) mutant lines but Pol II recruitment was not impaired, unlike the situation for CRT-containing genes. Expression of cold-inducible genes was not impaired in all of the Mediator tail subunit mutant lines tested; NRB4/MED15 mutants were not impaired in their expression of cold-inducible genes but preliminary result suggested that this subunit might be involved in UV-induced gene expression. In addition to their role in the transcriptional response to cold, both SFR6/MED16 and MED14 subunits were shown to be required for the expression of known CBF-controlled cold-inducible genes in response to sugar, but the MED2 subunit was not. Preliminary experiments conducted on CDK-8 domain subunit mutant lines, CYCC and CDK8, indicated that the CDK-8 domain may not function solely as a transcriptional repressor but may be required for expression of dark- and UV-inducible genes. Together, these data illustrate that transcriptional control in plants is achieved through the combined action of subsets of Mediator subunits that are defined by the stimulus and the particular gene investigated.