Gene expression in mitochondria:
The gene expression is studied in vertebrates mitochondria. It is divided into two steps i,e a) transcription and b) translation process.
A) Transcription process:
i)Initiation:
The transcription of each strand of vertebrate mt DNA starts at a site within D-loop(Displacement loop). The DNA strands at D-loop produce all kinds of RNAs, however rRNA produced at faster rate. In human and mouse mt DNA, two promoters for transcription occur within D-loop, separated from one another by 150 bps. These promoters are called H-strands promoter(HSP) and L-strand promoter(LSP). Each promoter is 50bp long and has two functional units, such as i) a short sequence called start site which is necessary for transcription initiation and ii) an upstream sequences located -12 to -39 bp upstream to start site, which increases efficiency of transcription initiation. A transcription factor called mt TF-1 binds to the sequence of upstream to start site and it has greater affinity for LSP than for HSP. The mt TF-1 activates transcription process by inducing physical changes(unwinding and bending of DNA) at the promoter site and and interacting with other proteins essential for transcription. The mitochondrial RNA P later binds at start site and initiates transcription.
ii) Elongation:
Nothing is known about the events associated with elongation of mitochondrial transcription process.
iii) Termination:
Although, nothing is known about elongation of transcription in mitochondria but some events studied the termination of transcription. A thirteen base pair sequence known as terminator sequence occur in the form of 5'-TGGCAGAGCCCGG-3'. This sequence is found to be sufficient for direct termination of transcription. A termination factor binds at the termination site and acts as a barrier to the transcription process, thus termination of transcription occurs.
B) Translation process:
Fallowing components of translation apparatus are found in mitochondria;
i) 70s ribosomes and tRNA specific to organelle
ii) other factors for translation
Initiation of translation takes place by formyl-met tRNA in mitochondria. Translation in mitochondria can be inhibited by chloramophenicol and not to cyclohexamide that inhibit 80s ribosomes. Proteins synthesized in mitochondria can be identified by inhibiting protein synthesis in cytoplasm by cyclohexamide. Mitochondrial DNA codes for two to three types of r RNA, 22-25 different tRNAs and thirteen to twenty six different protein molecules. Although some proteins are synthesized in mitochondria, but all proteins present in mitochondria are not synthesized by mitochondrial DNA alone. There are some mitochondrial proteins which are synthesized in cytoplasm and then transported to mitochondria.
There are some proteins of dual origin i,e some polypeptides produce by nuclear DNA and some polypeptides produce by mt DNA . The proteins which are synthesized in mitochondria are i) Cytochromes oxidase which has 7 kinds of polypeptides, of which three are synthesized in mitochondria and four are synthesized in cytoplasm. ii) Adenosine tri phosphatase(ATPase) has many polypeptides but mitochondrial DNA produce only two polypeptides. There for mitochondrial DNA produce about 10% of proteins for mitochondrial structure and functions and the remaining is coded by nuclear DNA.
The genetic code of mitochondria differs from that of nuclear RNA for example UGA codes for tryphtophan in mitochondria instead of termination and AUA codes for methionine instead of isoleucine(nuclear code).
