Fig. 32.2. A model of the structure of prokaryotic RNA polymerase showing association of five polypeptides (α2ββ').
Fig. 32.3. Active centres in bacterial RNA polymerase enzyme.
In bacterial systems like
E. coli a single RNA polymerase (RNAP) species is responsible for synthesis of all kinds of RNAs (mRNA, tRNAs and rRNA). This RNA polymerase has been purified and its structure and function is now known in some detail. It consists of five polypeptide chains including two chains of
alpha (α) polypeptide and one chain each of
beta (β
), beta dash (β
') and
sigma (σ) polypeptides (Fig. 32.2), the details for which are given in Table 32.1. The RNA polymerase molecule, thus can be represented as α
2ββ
'σ, in which attachment of sigma (σ) factor is not very firm, so that the core enzyme (α
2ββ
') can be easily isolated. The active sites of core enzyme are shown in Figure 32.3. Functions of different polypeptide chains are now understood, though not in any detail. For instance βand β
', which form the 'catalytic centre' of RNAP, help RNA polymerase in unwinding of DNA molecule for transcription. The sigma (σ) factor helps in recognition of start signals on DNA molecule and directs RNA polymerase in selecting the initiation sites. In the absence of sigma (σ), core enzyme initiates RNA synthesis in a random manner, suggesting the role of σ in recognition of initiation sites. Once RNA synthesis is initiated, σ dissociates after RNA is 8-9 bases long and then the core enzyme brings about elongation of mRNA. The dissociated sigma factor may again combine with core enzyme to form RNA polymerase holoenzyme (Fig. 32.4).
Fig. 32.4. Role of sigma factor Nus A, and core enzyme of RNA polymerase during transcription.
The α and β
' have constant sizes in most bacteria; the σ varies from 32,000 to 92,000.
Although, in prokaryotes like
E. coli, all RNA synthesis is done by only one kind of RNA polymerase molecules, there may be more than one sigma (σ) factors, which associate, each with the same core enzyme at different times for expression of different genes. For example, in
E. coli, besides σ
70 used under normal conditions of growth, atleast three other sigma factors (σ
32, σ
54, σ
28) are now known, which are used under adverse conditions like high temperature, nitrogen deficiency and for chemotaxis (consult
Regulation of Gene Expression 1. Operon Circuits in Bacteria and other Prokaryotes for more details).
Fig. 32.2. A model of the structure of prokaryotic RNA polymerase showing association of five polypeptides (α2ββ').
Fig. 32.3. Active centres in bacterial RNA polymerase enzyme.
Fig. 32.4. Role of sigma factor Nus A, and core enzyme of RNA polymerase during transcription.