Transcription in Plant Mitochondria

Abstract

Transcription in plant mitochondria is sustained by phage-type RNA polymerases that are encoded by a small nuclear encoded RpoT gene family (RNA polymerase of the T-phage type). In angiosperms, it consists of a minimum of two genes, encoding enzymes that are imported into mitochondria (RpoTm) and plastids (RpoTp). An additional gene for a dually targeted enzyme is found in eudicots (RpoTmp). The promoters recognized by mitochondrial RpoT enzymes contain a CRTA-, YYTA-, or DDTA-motif, often accompanied by an upstream A/T-rich region involved in modulating transcription. Transcription of most mitochondrial genes is driven by multiple promoters. Multiple promoters may ensure transcription despite possible mitochondrial genome rearrangements. Transcription factors have not been identified in plant mitochondria. Although proteins related to transcription factors in fungal and animal mitochondria (mtTFA and mtTFB) are encoded in the nuclear genome of Arabidopsis, none of these proteins has been shown to function as a factor assisting RNA polymerases in promoter recognition.

Glossary

Alloplasmic:

Describes an individual having a common nucleus, but an “alien” cytoplasm (e.g., alloplasmic rye containing a wheat cytoplasm); may lead to meiotic disturbances and male sterility (CMS).

Bacterial-type RNA polymerase:

RNA polymerase found in bacteria. The core enzyme has 4 subunits. α: two α-subunits assemble the enzyme and bind regulatory factors. β: catalyzes the RNA synthesis. β′: binds to DNA (nonspecifically). To bind promoter-specific regions, the core enzyme requires the sigma factor (σ), which together form the holoenzyme. Related RNA polymerases are found in plastids (β′ is split into two subunits in the cyanobacterial and plastid enzymes) and mitochondria of Jakobids.

Bryophyte:

Nonvascular, seedless plants. Bryophytes are among the simplest of the terrestrial plants and comprise mosses, hornworts, or liverworts; e.g., Physcomitrella, Marchantia.

Cis-element:

Defines a region of DNA or RNA that regulates the expression of genes located on the same strand. Cis-elements are often binding sites of one or more trans-factors.

CMS:

Cytoplasmic male sterility (see Sect.18.1).

GFP:

Green fluorescent protein; exhibits bright green fluorescence when exposed to blue light and is frequently used as a reporter of expression.

HSP:

Heavy-strand promoter; part of the bidirectional promoter in mammalian mitochondria initiating transcription of the heavy-strand (in opposite the light-strand, defined by their differing G + T content).

LSP:

Light-strand promoter; promoter in mammalian mitochondria initiating transcription of the light-strand.

Lycophyte:

Plantae subdivision comprising the oldest living vascular plants at around 410 million years old, e.g., Selaginella.

NEP:

Nuclear-encoded plastid RNA polymerase; a phage-type RNA polymerase that originated by gene duplication from the gene encoding the mitochondrial RNA polymerase.

PPR protein:

Pentatricopeptide repeat protein (see Chap. 4).

Promoter:

A region of DNA that facilitates the transcription of a particular gene. In plant mitochondria, most promoters were defined by mapping transcript initiation sites and identification of consensus motifs. Therefore, promoters are often specified by the gene’s name and position of the initiating nucleotide in respect to the start of the coding sequence or mature rRNA.

Prophage:

Latent form of a bacteriophage inserted as part of the chromosome of a bacterium.

Tracheophyte Vascular plants:

those plants that have lignified tissues for conducting water, minerals, and photosynthetic products. Tracheophytes include lycophytes, ferns, and seed plants.

Trans-factors:

Mostly a protein factor that binds to specific DNA or RNA sequences (cis-elements) thereby controlling the regulation/expression of a certain gene.