There are many structural genes associated in an operon system. These direct the synthesis of the mRNA and govern the sequence of amino acids in a protein molecule. Each structural gene might produce a particular kind of protein or all structural genes might regulate the production of a single protein. The activities of the structural gene (synthesis) are controlled by the promoter and operator of the operon system. The most well studied structural genes (z, y and a) are those of the Lac operon system in Escherichia coli.
The operator gene is situated adjacent to the first structural gene. It switches on or switches off the functioning of the structural gene (protein synthesis). In case a structural gene has to be suppressed, a repressor attaches itself to the operator to form an Operator – repressor complex.
In the case of protein synthesis, the operator – repressor complex prevents the transcription by blocking the movement of RNA polymerase. Promoter gene: The promoter gene is continuous with the operator gene and is believed to lie left to it.
It is suggested that RNA polymerase binds to the promoter site during transcription. Three regions have been recognised in the promoter site. These are a) recognition site, initial binding site and the mRNA initiation site (operator site).
(i) Recognition site:
Also called the cga site (catabolic gene activator site), it consists of certain palindromic sequnces of DNA. These symmetrical sections of DNA are recognised by proteins having symmetrically placed sub units.
This site, also called the CRP site (cyclic AMP receptor protein site) binds a CR protein to the promoter gene and thus facilitates the binding of the enzyme RNA polymerase. It has been found that in E.coli CRP combines with cAMP (cyclic adenosine monophosphate) forming a CRP – cAMP complex which binds to the promoter enchancing the binding of RNA polymerase and activates transcription. This regulation is called positive control.
(ii) Initial binding site:
This consists of seven bases (DNA) to which the RNA polymerase binds.
(iii) RNA initiation site:
The site where transcription begins is called initiation site. This is the region overlapping with the operator region. Regulator gene: The regulator gene directs the activity of the operator gene by producing inhibitor proteins called repressors. This repressor protein binds to the operator gene and blocks the path of RNA polymerase, thus preventing transcription. If an inducer is present in the system, it binds to the repressor which undergoes conformational change and becomes inactive. As the inactive repressor cannot bind to the promoter, the strucutral genes get activated and protein synthesis continues.
Briefly explain the modern concept of protein synthesis? Modern concept of protein synthesis. The biogenetic process which assembles and links the specific amino acids in specific sequence to form protein molecules is depicted as the central dogma of protein synthesis. Replication Transcription Translation
At the time of protein synthesis under the activity of certain enzymes the DNA helix unwinds and separates. One of the DNA strand acts as the template and synthesises a particular RNA strand as its complimentary strand by attracting the 4 types of ribonucleotides found in the nuclear medium.
The base sequence on the RNA strand is complimentary and determined by those on the DNA strand. In the process, the genetic message of the DNA (base sequence) is transcribed on the m RNA strand. This is called transcription and is catalysed by an enzyme, DNA dependent RNA Polymerase.
The mRNA with transcribed genetic code diffuses out of the nucleus and arrives at the site of protein synthesis – the ribosomes. The separated DNA strands recoil to form the double helix.
Translation of the genetic code takes place by the coordination of ribosomes, mRNA, rRNA, tRNA amino acids and enzymes. The mRNA strand runs through a cluster of 7 or more activated ribosomes, forming polyribosomes.
Meanwhile the amino acids in the cytoplasm get activated and charged with energy. This is promoted by ATP. The activated amino acids become chemically bound with specific tRNA at its end under the action of an enzyme. This is called amino acid tRNA complex. Many such ‘complexes’ are transferred to the polyribosomes.
Each tRNA has a ‘binding site’ bearing triplet bases called the anticodons, which are complimentary to specific triplet codon, on the RNA. Chemical binding occurs between the complimentary codon of mRNA and anticodon of different tRNA complexes.
As the ‘ b i n d i n g’ goes on, the amino acids carried by each tRNA link with one another forming a peptide chain of protein molecule. By the time the mRNA passes off the last ribosome of the polyribosome, the mRNA and peptide chain of protein are released under the action of R1 and R2 factors. The ribosome and tRNAs are also released.