Biotechnology and Bioinformatics (1+1)

1.3.4. Gene expression in prokaryotes

An average bacterium, contains one thousands the DNA content of a typical eukaryotic cell. The bacterial chromosome contains a single circular DNA molecule associated with a few proteins and is not enclosed within a limiting membrane unlike that in the eukaryotic cell. Bacteria can divide very rapidly. The doubling time is also referred to as generation time and in some bacteria, this can be as low as 20 minutes from a single origin of replication and can proceed bidirectionally.

The bacterium Escherichia coli has about 2,500 genes. The expression of these genes is usually controlled to achieve maximum cellular economy. This means that genes will be turned on or off as per the requirement. A set of genes will be switched on when there is necessity to handle and metabolise a new substrate. When these genes are turned on, enzymes are produced, which metabolise the new substrate. The phenomenon is known as induction and the small molecules eliciting this induction are referred to as inducers.

Similarly, when a metabolite needed by the bacterium is provided in excess from outside, the bacterium stops making it and thus conserves its reserves. This is achieved by the added metabolite turning off a set of genes involved in producing that metabolite in the bacterial cell. This phenomenon is known as feed back repression.

As against the processes of induction and repression as already indicated a set of genes are constantly expressed to take care of house keeping functions such as glycolysis. These genes which are constantly expressed are referred to as constitutive.

In 1961, Francois Jacob and Jacques Monod, at the Pasteur Institute in Paris, proposed that metabolic path ways are regulated as a unit. For example, when the sugar lactose is added to the cultures of E. coli, it induces three enzymes necessary to break down the lactose into glucose and galactose.