REGULATION OF GENE EXPRESSION
These regulatory proteins can act both positively (activators) and negatively (repressors). The accessibility of promoter regions of prokaryotic DNA is in many cases regulated by the interaction of proteins with sequences termed operators. The operator region is adjacent to the promoter elements in most operons and in most cases the sequences of the operator bind a repressor protein.
Each operon has its specific operator and specific repressor. For example, lac operator is present only in the lac operon and it interacts specifically with lac repressor only.
The Lac Operon
Jacob and Monod (1961) proposed a model of gene regulation, known as operon model. Operon is a co-ordinated group of genes such as structural genes, operator genes, promoter genes, regulator genes and repressor which function or transcribed together and regulate a metabolic pathway as a unit.
There are three structural genes, lac Z, Y and lac A, coding for galactosidase, permease and transacetylase respectively. These three genes are controlled by a single switch called operator. The operator switch is controlled by the repressor protein which coded by the regulator gene.
When the repressor binds to the operator, the genes are not expressed (switched off). When the operator switch is on, the three structural genes transcribe a long polycistronic mRNA catalysed by RNA polymerase.
A few molecules of lactose (inducer) enter the cell by the action of enzyme permease. They are converted into an active from of lactose which binds to the repressor and changes its configuration and prevents it from binding to the operator. Beta-galactosidase breaks lactose into glucose and galactose.
The process of translation requires transfer of genetic information from a polymer of nucleotides to a polymer of amino acids. It was George Gamow, a physicist, who argued that since there are only 4 bases and if they have to code for 20 amino acids, the code should constitute a combination of bases. He suggested that in order to code for all the 20 amino acids, the code should be made up of three nucleotides. This was a very bold proposition, because a permutation combination of 43 (4x4x4) would generate 64 codnos; generating many more codons than required.
The chemical method developed by HarGobind Khorana was instrumental in synthesizing RNA molecules with defined combination of bases (homopolymers and co-polymers). Marshall Nirengerg’s cell-free system for protein synthesis finally helped the code to be deciphered.
Severo Ochoa enzyme (polynucleotide phosphorylase) was also helpful in polymerizing RNA with defined sequences in a temple independent manner (enzymatic synthesis of RNA).
The salient features of genetic code are as follows :
(i) The codon is triplet. 61 codons code for amino acids and 3 codons do not code for any amino acid, hence they function as stop codons.
(ii) One codon codes for only one amino acid, hence, it is unambiguous and specific.
(iii) Some amino acids are coded by more than one codon, hence the code is degenerate.
(iv) The codon is red read in mRAN in a contiguous fashion. There are no punctuations.
(v) The code is nearly universal : for example, from bacteria to human UUU would code for Phenylalanine (phe). Some exceptions to this rule have been found in mitochondrial codons, and in some protozoans.
(vi) AUG has dual functions. It codes for Methionine (met), and it also act as initiator codon.