The leader sequence in the polycistronic mRNA of the here, but each of the 5 has similar mechanisms for trp operon contains several trp codons, and can form 3 initiation, elongation, and termination of transcription. different stem-loop structures. Depending on the Eukaryotic are nearly always monocistronic amount of available tryptophan, one of two structures mRNAs with a general structure as shown in Figure can be produced (Figure 3.29) One structure to 3.30. The key transcribed features are a 5’-UTR termination of transcription in the leader sequence (untranslated region), a coding region, and 3’-UTR. when trp is abundant. While the other structure does Other non-trans-cribed features that are typical of not terminate transcription, and the polycistronic mRNA mRNAs in is produced. Several amino acid synthetic operons (e.g. histidine, leucine, threonine, and Eukaryotic cells include a 5’-Cap structure and a poly-A demonstrate this same type of attenuation. tail that will be described in more detail below. Consequently, this mechanism is relatively widespread Promoters in many Eukaryotes have been analyzed as a means of modulating and fine-tuning pathways for either by the use of directed mutations within promoter amino acid biosynthesis. sequences or by comparative analysis of multiple genes 3.5.6. Transcription in Eukaryotes from different organisms. These studies have revealed Although transcription in Eukaryotes follows the that there are two types of elements found in general principles outlined above for Prokaryotes, there Eukaryotic promoters, core promoter elements and are many specific details that are different. Recall that promoter proximal elements. there are as many as five . While each of these Core promoter elements are located near the transcribes different types of RNA, they are all transcription start site and specify where transcription Multisubunit RNA polymerases that function in related begins. Examples include: ways. The mechanism of the important RNA The initiator element (Inr), a pyrimidine-rich A polymerase II that produces mRNAs will be described that spans the transcription start site;
The leader sequence in the polycistronic mRNA of the trp operon contains several trp codons, and can form 3 different stem-loop structures. Depending on the amount of available tryptophan, one of two structures can be produced (Figure 3.29) One structure to termination of transcription in the leader sequence when trp is abundant. While the other structure does not terminate transcription, and the polycistronic mRNA is produced. Several amino acid synthetic operons (e.g. histidine, leucine, threonine, and demonstrate this same type of attenuation. Consequently, this mechanism is relatively widespread as a means of modulating and fine-tuning pathways for amino acid biosynthesis. 3.5.6. Transcription in Eukaryotes Although transcription in Eukaryotes follows the general principles outlined above for Prokaryotes, there are many specific details that are different. Recall that there are as many as five . While each of these transcribes different types of RNA, they are all Multisubunit RNA polymerases that function in related ways. The mechanism of the important RNA polymerase II that produces mRNAs will be described here, but each of the 5 has similar mechanisms for initiation, elongation, and termination of transcription. Eukaryotic are nearly always monocistronic mRNAs with a general structure as shown in Figure 3.30. The key transcribed features are a 5’-UTR (untranslated region), a coding region, and 3’-UTR. Other non-trans-cribed features that are typical of mRNAs in Eukaryotic cells include a 5’-Cap structure and a poly-A tail that will be described in more detail below. Promoters in many Eukaryotes have been analyzed either by the use of directed mutations within promoter sequences or by comparative analysis of multiple genes from different organisms. These studies have revealed that there are two types of elements found in Eukaryotic promoters, core promoter elements and promoter proximal elements. Core promoter elements are located near the transcription start site and specify where transcription begins. Examples include: The initiator element (Inr), a pyrimidine-rich A that spans the transcription start site;