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GENE REGULATION IN EUKARYOTES

In eukaryotes, only 2-15% of the genes are expressed at a particular time and in comparison to the prokaryotes, eukaryotes have more complex gene regulation systems, like:

1.Availability of Many Types of RNA Polymerases. In these organisms, a gross level of gene expression regulation is found by having three types of RNA polymerases in the cell :

(a) RNA polymerase I, synthesizes rRNA.

(b) RNA polymerase II, synthesizes hnRNA or mRNA.
(c) RNA polymerase III, synthesizes tRNA.

In the cell organelles, mitochondria and chloroplasts, only one type of RNA polymerase, like in prokaryotes, synthesizes all the three types of RNAs.

 

2.Chromatin Proteins. Chromatin in eukaryotes is made up of:

(a) DNA

(b) Histones

(c) Non-histones
(d) RNA

More specific regulation of gene expression is exercised by the chromatin protein histones and non-histones.

Histones are highly conserved and same five types of histones, H1, H2A, H2B, H3 an H4, are found in all the primitive and advanced eukaryotes. Histones repress the gene activity non-specifically and as long as they are associated with the DNA and they repress the activity of the genes. Thus, for RNA synthesis in nucleus, these histones have to be separated from the DNA temporarily.

Non-histones are highly variable and they vary not only from one species to other species, but also from tissue to tissue in the same organism. Unlike histones, which are same
in all the types of tissues, non-histones, being variable, give rise to different types of mRNAs in different tissues. Gilmour and Paul (1973) performed the chromatin reconstitution experiments in mammals and showed that non-histones playa positive role in gene expression. They isolated the chromatin from different types of tissues, e.g., thymus and bone marrow, and separated their constituents, DNA, histones and non-histones.

Thereafter, they reconstituted all the possible types of chromatins by recombining the various chromatin components in different combinations. The type of mRNA produced from these artificially reconstituted chromatin depended upon the non-histones and not upton the DNA and histones, Thus, irrespective of the DNAs and histones, the reconstituted chromatins that contained the thymus type of non-histones gave rise to the thymus-type of mRNAs, whereas those containing bone marrow type non-histones gave bone marrow type of mRNAs (Fig, 11.11 see Appendix),

2.Regulation at the Transcriptional Level. For regulation of gene expression at the mRNA synthesis level in eukaryotes, Britten and Davidson (1969) proposed a ‘Gene-battery model’ or ‘Britten-Davidson model’, which is based on the presence of a large proportion of repetitive DNA in eukaryotes. (Those DNA sequences that are found in single copy are known as unique sequences, whereas the DNA sequences that are repeated many times or found in multiple copies are called repetitive or redundant DNA), These repetitive DNA sequences may play some regulatory role in gene expression.

The ‘Britten-Davidson model’ has:

    1. A sensor site, which regulates the activity of integrator gene. The integrator gene is transcribed only when the sensor site is activated by some hormones and proteins.
    2. An integrator gene that is equivalent to the regulator gene i of operons in prokaryotes. Under the control of sensor site, it produces an activator RNA (or protein) that activates the receptor site.
    3. A receptor site, which is equivalent to operator site in bacterial operons, is found adjacent to the producer gene.
    4. Structural gene is similar to the structural genes of operons and gives rise to the mRNA (Fig 11.12)Fig.11.12

Fig 11.12 Britten and Davidson’s model of regulations of gene expression.

It has been proposed that the presence of multiple copies of receptor sites and sensor sites may control the expression of genes. Thus, according to this model, a battery is made up of:

In this way, all the structural genes controlled by a sensor make a battery. The repetitive receptor sites, thus, may control the gene expression as (Fig. 11.13): 

Fig.11.13

Fig 11.13 Role of repetitive of receptor sites in controlling the gene expression in eukaryotes

Similarly, the repetitive integrator genes may also help in the regulation of gene expression (Fig.11.14)

Fig.11.14

Fig, 11.14 Role of repetitive integrator genes in controlling
the gene expression in eukaryotes.

 4.Regulation at the Translational Level. In many amphibians, the unfertilized egg contains some mRNAs that remain untranslated until it is fertilized by a sperm, e.g., in Xenopus translation of mRNA for fibronectin protein occurs after 6-7 hours of fertilization. Thus, fertilization, here, controls the gene expression at translation level.

QUESTIONS

  1. Describe lac operon in E. coli.
  2. Discuss a repressible system of gene expression regulation in prokaryotes.
  3. With the help of diagrams, explain the role of leader sequence in controlling the gene expression in E coli.
  4. How histones and non-histones are involved in regulation of gene expression in eukaryotes?
  5. Explain gene-battery model of gene regulation.
  6. Discuss the role of repetitive DNA in gene regulation in eukaryotes.