Termination of Transcription in Bacteria

Much less is known about termination of transcription in eukaryotes than in bacteria. Once transcribed, specific termination sequences of mRNA are bound by protein complexes (such as CPSF cleavage and poly(A)denylation specificity factor, and CstF cleavage stimulation factor) that process cleavage of the message and subsequent poly(A)denylation. RNA polymerase continues transcription for up to several hundred nucleotides before dissociating from the DNA. 

Polycistronic mRNA, which forms as a primary transcript in bacteria, is a continuous length of RNA transcribed from a single promoter. It will therefore contain only one normal termination sequence (i.e.. ignoring a possible attenuator sequence before the first initiation codon). 

Riboswitch-mediated gene regulation occurs after transcription initiation, as the regulatory element is part of the regulated transcript. In bacteria, preferred modes of regulation are at the levels of premature termination of transcription (referred to as transcriptional attenuation) and translation initiation. Rare examples identified so far in eukaryotic cells affect mRNA stability and splicing. 

The function of the leader sequence is to fine tune expression of the trp operon based on the availability of tryptophan inside the cell. It does this as follows. The leader sequence contains four regions (Fig. 2, numbered 1-4) that can form a variety of base-paired stem-loop ( hairpin ) secondary structures. Now consider the two extreme situations the presence or absence of tryptophan. Attenuation depends on the fact that, in bacteria, ribosomes attach to mRNA as it is being synthesized and so translation starts even before transcription of the whole mRNA is complete. When tryptophan is abundant (Fig. 2a), ribosomes bind to the trp polycistronic mRNA that is being transcribed and begin to translate the leader sequence. Now, the two trp codons for the leader peptide lie within sequence 1, and the translational Stop codon (see Topic HI) lies between sequence 1 and 2. During translation, the ribosomes follow very closely behind the RNA polymerase and synthesize the leader peptide, with translation stopping eventually between sequences 1 and 2. At this point, the position of the ribosome prevents sequence 2 from interacting with sequence 3. Instead sequence 3 base-pairs with sequence 4 to form a 3 4 stem loop which acts as a transcription terminator. Therefore, when tryptophan is present, further transcription of the trp operon is prevented. If, however, tryptophan is in short supply (Fig. 2b), the ribosome will pause at the two trp codons contained within sequence 1. This leaves sequence 2 free to base pair with sequence 3 to form a 2 3 structure (also called the anti-terminator),... 

Where does transcription of a DNA template end The process of transcription termination is best understood in bacteria. Rho-independent terminators are characterized by a self-complementary... 

Bicyclomycin is certainly the most important compound to be cited in this section. Because of its broad spectrum of activity displayed against Gram-negative bacteria (such as Escherichia coli, Klebsiella, Salmonella, Shigella, and Citrobacter), coupled with its low toxicity, bicyclomycin was introduced into the market under the trade name Bicozamycin. The mode of action of bicyclomycin was studied in the 1980s and early 1990s, and only information concerning its locus of action in E. coli, that is, the rho transcription termination factor (rho is a protein responsible for termination of RNA... 

Transcription usually ends within or after pyrimidine-rich regions [120,141,145,150]. Among the methanogens, inverted repeats that might form stem-loop structures, reminiscent of the rho-independent terminators of bacteria, have been identified close to the termination sites [100]. Usually, there are multiple transcription stops in the archaea. 

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