E. coli RNA polymerase holoenzyme

The binding of E. coli RNA polymerase holoenzyme to bacterial promoters, and the subsequent initiation of RNA synthesis, has been studied in detail and a general scheme of these events is shown in Figure 24.2. 

RapA RapA E. coli RNA polymerase core or RNA polymerase holoenzyme Recycling of RNA polymerase [332]. 

FIGURE 26-4 Structure of the RNA polymerase holoenzyme of the bacterium Thermus aquaticus. (Derived from PDB ID 1 IW7.)The overall structure of this enzyme is very similar to that of the E. coli RNA polymerase no DNA or RNA is shown here. The j3 subunit is in gray, the j3 subunit is white the two a subunits are different shades of red the to subunit is yellow the cr subunit is orange. The image on the left is oriented as in Figure 26-6. When the structure is rotated 180° about the y axis (right) the small to subunit is visible. 

Transcription cycle of E. coli RNA polymerase showing dissociation of the ct subunit shortly after chain elongation begins, dissociation of the core enzyme during termination, and re-formation of the holoenzyme from the core enzyme and the a subunit. A previously joined core enzyme and a subunit will rarely become rejoined instead, reassociation occurs at random. 

We begin our consideration of transcription by examining the process in bacteria such as E. coli. The E. coli RNA polymerase is a very large (MOO kd) and complex enzyme consisting of four kinds of subunits (Table 29.1). The subunit composition of the entire enzyme, called the holoenzyme, is a >(3p a. The (T subunit helps find a promoter site where transcription begins, participates in the initiation of RNA synthesis, and then dissociates from the rest of the enzyme. RNA polymerase without this subunit (a2PP ) is called the core enzyme, which contains the catalytic site. 

RNA synthesis requires DNA-dependent RNA polymerase holoenzymes. The E. coli RNA polymerase consists of five subunits, one of which, the s subunit, is only used during initiation for promoter recognition. Eukaryotes have three different RNA polymerase holoenzymes called RNA pol I, RNA pol II, and RNA pol III. 

Subunits - E. coli RNA polymerase is a multi-subunit protein. The five distinct polypeptide subunits of E. coli RNA polymerase are summarized in Table 26.1, Two copies of the ot subunit are present, along with one each of /, [Pg.110]

Correct transcription of phage DNA by E. coli RNA polymerase needs holoenzyme (Burgess et al., 1969 Hinkle and Chamberlin, 1970). In... 

FIGURE 26-5 Typical E. coli promoters recognized by an RNA polymerase holoenzyme containing a70. Sequences of the nontemplate strand are shown, read in the 5 —>3 direction, as is the convention for representations of this kind. The sequences vary from one promoter to the next, but comparisons of many promoters reveal similarities, particularly in the —10 and -35 regions. The sequence element UP, not present in all E. coli promoters, is shown in the P1 promoter for the highly expressed rRNA gene rrnB. UP elements, generally occur-... 

E. coli RNA Pol is probably the best characterized among all known prokaryotic RNA polymerases. An E. coli cell contains 3000 RNA polymerase molecules. The holoenzyme (—480 kDa) consists of five types of subunits ()8, /3, a, w, and O ) which can be separated into two components the core enzyme a2 P ai) and the a- factor (cr protein). The core enzyme is catalytically competent, but binds nonspecifically to DNA. The cr factor, which alone does not bind DNA, confers the promoter-binding specificity to the core enzyme. The elongation of the chain is about 40 nucleotides/sec at 37°C or 13—20 nucleotides/sec at 20°C (2). 

The RNA polymerase of E. coli possesses with its subimit construction (a2PP o) a simple structure in comparison to eucaryotic RNA polymerases. The sigma factor is only required for the recognition of the promoter and the subsequent formation of a tight complex. After the incorporation of the first 8-10 nucleotides into the transcript, the sigma factor dissociates from the holoenzyme, and the remaining core enzyme carries out the rest of the elongation. 

In E. coli, all genes are transcribed by a single large RNA polymerase with the subunit structure a2pp a. This complete enzyme, called the holoenzyme, is needed to initiate transcription since the a factor is essential for recognition of the promoter it decreases the affinity of the core enzyme for nonspecific DNA binding sites and increases its affinity for the promoter. It is common for prokaryotes to have several a factors that recognize different types of promoter (in E. coli, the most common a factor is a70). 

RNA polymerase in h. coli is a multisubunit enzyme. The subunit composition of the ". SOO-kd holoenzyme is u >3 3 a and that of the core enzyme is ajpp. IVanscription is initiated at promoter sites consisting of two sequences, one centered near —10 and the other near — TS that is, 10 and d5 nucleotides away from the start site in the 5 (upstream) direction. The consensus sequence of the —10 region is TATA AT The a subunit enables the holoenzyme to recognize promoter sites. When the growth temperature is raised. E. coli expresses a special ex subunit that selectively binds the distinctive promoter of lieat-shock genes, RNA polymerase must unwind the... 

The enzyme that synthesizes RNA is called RNA polymerase, and the most extensively studied one was isolated from E. coli. The molecular weight of this enzyme is about 470,000 Da, and it has a multisuhunit structure. Five different types of suhunits, designated a, CO, P, P, and o, have heen identified. The actual composition of the enzyme is 0t2(0pP o. The o-suhunit is rather loosely hound to the rest of the enzyme (the 0t2(0pp portion), which is called the core enzyme. The holoenzyme consists of all the suhunits, including the a-suhunit. 

Give the subunit composition of the RNA polymerase of E. coli for both the holoenzyme and the core enzyme. 

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