Transcription in prokaryotes

The most detailed molecular information about the transcription cycle is available in bacterial systems. The synthesis of RNA is initiated at the promoter sequence by the enzyme RNA polymerase. A single RNA polymerase type is responsible for the synthesis of messenger, transfer, and ribosomal RNAs. 

When isolated from bacteria, prokaryotic RNA polymerase has two forms The core enzyme and the holoenzyme. The core enzyme is a tetramer whose composition is given as 0C2PP (two alpha subunits, one beta subunit, and one beta-prime subunit). Core RNA polymerase is capable of faithfully copying DNA into RNA but does not initiate at the correct site in a gene. That is, it does not recognize the promoter specifically. Correct promoter recognition is the function of the holoenzyme form of RNA polymerase. 

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Comparative processing of major transcripts in prokaryotes and eukaryotes. 

In the previous chapter we discussed DNA-binding proteins that regulate transcription in prokaryotes. The principles that govern recognition between proteins in eukaryotes show some similarities and some differences. In both cases specific recognition is dominated by interactions that take place in the major groove of the DNA. The specific interactions usually involve H bond formation... 

Further details of the nature and regulation of transcription in prokaryotes and eukaryotes and the post-transcriptional processing of mRNA transcripts are given in Chapter 9. 

We have seen how interactions between DNA-binding proteins such as CAP and RNA polymerase can activate transcription in prokaryotic cells (Section 31.1.6). Such protein-protein interactions play a dominant role in eukaryotic gene regulation. In contrast with those of prokaryotic transcription, few eukaryotic transcription factors have any effect on transcription on their own. Instead, each factor recruits other proteins to build up large complexes that interact with the transcriptional machinery to activate or repress Panscription. 

A. Rifampidn binds to RNA polymerase and prevents initiation of transcription in prokaryotes. 

DNA stores genetic information in a stable form that can be readily replicated. However, the expression of this genetic information requires its How from DNA to RNA to protein, as was introduced in Chapter 4, The present chapter deals with how RNA is synthesized and then modified to prepare for its translation into protein. We begin with transcription in prokaryotes and focus on the three stages of transcription promoter binding and initiation, elongation of the nascent RNA transcript, and termination at the end of the gene. 

Precursors of Transfer and Ribosomal RNA Are Cleaved and Chemically Modified After Transcription in Prokaryotes... 

Eukaryotes utilize many more initiation factors than do prokaryotes, and their interplay is much more intricate. The prefix elF denotes a eukaryotic initiation factor. For example, eIF-4E is a protein that binds directly to the 7-inethylguanosine cap (p. 846), whereas eIF-2, in association with GTP, delivers the met-tRNA to the ribosome. The difference in initiation mechanism between prokaryotes and eukaryotes is, in part, a conseciuence of the ence in RNA processing. The 5 end of mRNA is readily available to ribosomes immediately after transcription in prokaryotes. In contrast, pre-mRNA must be processed and transported to the cytoplasm in eukaryotes before translation is initialed. The 5 cap provides an easily recognizable starting point. In addition, the complexity of eukaryotic translation initiation provides another mechan ism for regulation of gene expression that we shall explore further in Chapter 31. 

Transcription in Prokaryotes Transcription in Eukaryotes GENE EXPRESSION... 

Although many differences exist between transcription in prokaryotes and eukaryotes, and even differences between transcription of different types of RNA in eukaryotes, some aspects are constant. Table 11.1 summarizes the main features of the process. 

The biggest difference between transcription in prokaryotes and eukaryotes is the sheer number of proteins associated with the eukaryotic version of the process. Any protein that regulates transcription but is not itself a subunit of RNA polymerase is a transcription factor. There are many transcription factors for eukaryotic transcription, as we shall see. The molecular mass of the entire complex of Pol II and all of the associated factors exceeds 2.5 million Da. 

How does RNA polymerase know where to begin transcription In prokaryotic transcription, RNA polymerase is directed to the gene to be transcribed by the interactions between the polymerase s O-subunit and sequences of DNA near the start site called promoters. Gonsensus sequences have been established for prokaryotic promoters, and the key elements are sequences at —35 and —10, the latter called the Pribnow box. In eukaryotic transcription, RNA polymerase binds to promoters as well, but there is no O-subunit, although there is a specific subunit, RBP4, that is involved in promoter recognition. 

Recall What are some of the main differences between transcription in prokaryotes and in eukaryotes ... 

Reflect and Apply How does regulation of transcription in eukaryotes differ from regulation of transcription in prokaryotes ... 

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