University:
Kentucky State University
Course:
BIO 408 | Cell Biology
Academic year:

2024
Views:

164

Pages:

10
Author:

Prince Sampson

Prokaryotic genes (operon) In bacteria, genes can be polycistronic. In bacteria, genes that encode enzymes, which are involved in related functions, often are located next to each other Example: the genes encoding the enzymes required to synthesize tryptophan are located in one contiguous stretch. This cluster of genes comprises a single transcriptional unit referred to as an operon. The RNA polymerase E. coli RNA polymerase is made up of multiple polypeptide chains. The core polymerase consists of two , one , one ’, and one  subunits. The core polymerase is fully capable of catalyzing the polymerization of NTPs into RNA. The  subunit is not required for the basic catalytic activity of the enzyme. Consensus sequences (the promoter) The DNA sequence to which RNA polymerase binds to initiate transcription of a gene is called the promoter. A promoter is "upstream" of the transcription initiation site. The region upstream of the transcription initiation site contains two sets of sequences that are similar in a variety of genes. They are called the (-10) and (-35) elements because they are located approximately 10 and 35 base pairs upstream of the transcription start site. The transcription initiation site is defined as the +1 position. Role of the  subunit In the absence of  , RNA polymerase binds to DNA with low affinity and nonspecifically. The role of  is to identify direct the polymerase to the -35 and -10 sequences. Mechanism of transcription (initiation) The he polymerase binds to the promoter and opens it. Single-stranded DNA is available as a template. Transcription is initiated by the joining of two NTPs. After addition of about the first 10 nucleotides,  is released from the polymerase.  RNA pol Closed-promoter complex Open-promoter complex Initiation of mRNA synthesis Release of sigma subunit Mechanism of transcription (elongation) As the polymerase moves forward, it unwinds the template DNA ahead of it elongates the RNA rewinds the DNA behind it Mechanism of transcription (termination) RNA synthesis continues until the polymerase encounters a termination signal where the RNA is released from the polymerase, and the enzyme dissociates from its DNA template. 20 Termination sequences The simplest and most common type of termination signal in E. coli consists of a symmetrical inverted repeat of a GC-rich sequence followed by A residues. Transcription of the GC-rich inverted repeat results in the formation of a stable stem-loop structure. The effect of the stem loop structure The formation of this structure breaks RNA association with the DNA template, destabilizes the RNA polymerase binding to DNA, and terminates transcription.

Transcription in Prokaryotes

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