High impact information on IM

• The data presented here are consistent with a role for the IM protein as a cofactor for carotenoid desaturation [1].
• We show that the IM protein is synthesized as a precursor polypeptide that is imported into chloroplasts and inserted into the thylakoid membrane [1].
• We propose a model in which IM function is linked to phytoene desaturation and, possibly, to the respiratory activity of the chloroplast [1].
• We have positionally cloned IM and found that the gene encodes a 40.5-kD protein with sequence motifs characteristic of alternative oxidase, a mitochondrial protein that functions as a terminal oxidase in the respiratory chains of all plants [2].
• We have cloned the IM gene by transposon tagging and show that even stable null alleles give rise to a variegated phenotype [1].

Biological context of IM

• We also examined the significance of a conserved sequence in (or near) the PTOX active site that corresponds precisely to Exon 8 of the IM gene [3].
• In this paper, we show that IM has a global impact on plant growth and development and is required for the differentiation of multiple plastid types, including chloroplasts, amyloplasts, and etioplasts [4].
• Sequences required for the activity of PTOX (IMMUTANS), a plastid terminal oxidase: in vitro and in planta mutagenesis of iron-binding sites and a conserved sequence that corresponds to Exon 8 [3].
• The immutans (im) variegation mutant of Arabidopsis has green and white leaf sectors due to the action of a nuclear recessive gene, IMMUTANS (IM) [4].
• Leaf anatomy is radically altered in the green and white sectors of im: Mesophyll cell sizes are dramatically enlarged in the green sectors and palisade cells fail to expand in the white sectors [4].

Anatomical context of IM

• The expression of photosynthetic genes is not appreciably altered in the green im sectors versus wild type, but rather there is an up-regulation of genes involved in defense against oxidative stress and down-regulation of genes involved in cell wall biosynthesis [5].

Associations of IM with chemical compounds

• The albino sectors of im plants contain reduced levels of carotenoids and increased levels of the carotenoid precursor phytoene [1].
• The thylakoid membranes of most photosynthetic organisms contain a terminal oxidase (PTOX, the product of the Arabidopsis IMMUTANS gene) that functions in the oxidation of the plastoquinone pool [3].
• PTOX and AOX are diiron carboxylate proteins, and based on crystal structures of other members of this protein class, a structural model of PTOX has been proposed in which the ligation sphere of the diiron center is composed of six conserved histidine and glutamate residues [3].
• Overexpression of IM did not result in increased capacity to keep the PQ pool oxidized compared to either the wild type or im grown under control conditions (25 degrees C and photosynthetic photon flux density of 150 mumol photons m(-2) s(-1)) [6].
• In order to determine whether the newly discovered plastid oxidase (PTOX) involved in carotenoid biosynthesis acts as a plastoquinol oxidase in higher plant chloroplasts, the Arabidopsis thaliana PTOX gene (At-PTOX) was expressed in tobacco under the control of a strong constitutive promoter [7].

Other interactions of IM

• The IMMUTANS variegation locus of Arabidopsis defines a mitochondrial alternative oxidase homolog that functions during early chloroplast biogenesis [2].

Analytical, diagnostic and therapeutic context of IM

• We tested the functional significance of these residues by site-directed mutagenesis of PTOX in vitro and in planta, taking advantage null immutans alleles for the latter studies [3].
• In contrast to AOX1a, meta-analyses of published Arabidopsis microarray data indicated that IM expression exhibited minimal modulation in response to myriad abiotic stresses, which is consistent with our functional data [6].
• Sequence analysis revealed that the amino-acid sequence of OsIM1 showed 66% and 62% identity with PTOX from tomato ( Capsicum annuum) and AtIM from Arabidopsis, both of which encoded chloroplast-orientated terminal oxidase [8].

References