Note(s): |
1When the CTP level within the cell is high relative to the level of GTP, the pyrD gene is transcribed from a start site that allows formation of a secondary structure within the 5' end of the mRNA. This hairpin structure is thought to obscure the Shine-Dalgarno sequence, and it results in poor translation efficiency. When the CTP level is low relative to the level of GTP, pyrD is transcribed from an alternate start site. These shorter mRNAs are translated efficiently. The PurR protein, which represses the pyrD gene, is also regulated by the level of GTP, because this protein is active when GTP is present, but when PurR is active it repress the two genes, and when there is a high level of GTP these genes are translated efficiently. Thus, the two control systems can operate in opposition to one another.1When the CTP level within the cell is high relative to the level of GTP, the pyrD gene is transcribed from a start site that allows formation of a secondary structure within the 5' end of the mRNA. This hairpin structure is thought to obscure the Shine-Dalgarno sequence, and it results in poor translation efficiency. When the CTP level is low relative to the level of GTP, pyrD is transcribed from an alternate start site. These shorter mRNAs are translated efficiently. The PurR protein, which represses the pyrD gene, is also regulated by the level of GTP, because this protein is active when GTP is present, but when PurR is active it repress the two genes, and when there is a high level of GTP these genes are translated efficiently. Thus, the two control systems can operate in opposition to one another.
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