High impact information on GUN5

• Arabidopsis genomes uncoupled 5 (GUN5) mutant reveals the involvement of Mg-chelatase H subunit in plastid-to-nucleus signal transduction [1].
• The comparison of mutant phenotypes of gun5 and another Mg-chelatase subunit (ChlI) mutant suggests a specific function for ChlH protein in the plastid-signaling pathway [1].
• GUN5 is the ChlH subunit of Mg-chelatase, which produces Mg-Proto, and GUN4 is a regulator of ChlH activity (2) [2].
• Finally, post-transcriptional up-regulation of the level of the CHLH subunit of the magnesium chelatase has been detected in the chlm mutant and most likely corresponds to specific accumulation of this protein inside plastids [3].
• Furthermore, although Lhcb and HEMA1 were responsive to both GUN1 and GUN5 signals, the relative inputs from these pathways differed for each transcript with GUN1 being dominant for HEMA1 regulation [4].

Anatomical context of GUN5

• The nuclear genes Lhcb and HEMA1 are differentially sensitive to plastid signals and suggest distinct roles for the GUN1 and GUN5 plastid-signalling pathways during de-etiolation [4].
• In higher plants, CCH has to be transported to specialised cells through plasmodesmata, being the only metallochaperone reported to date that leaves the cell where it is synthesised [5].

Associations of GUN5 with chemical compounds

• We previously identified from broad bean an ABA-binding protein (ABAR) potentially involved in stomatal signalling, the gene for which encodes the H subunit of Mg-chelatase (CHLH), which is a key component in both chlorophyll biosynthesis and plastid-to-nucleus signalling [6].

Other interactions of GUN5

• Strikingly, CHLH and CHLI-2 transcripts were also absent [7].
• These results can be interpreted as a metabolic (rather than genetic) interaction between HY1 and GUN4 or GUN5, and this in turn supports the involvement of tetrapyrroles as plastid signals [8].

References