However - complementary GC-rich sequence and prevent...

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However, under low levels of tryptophan premature termination is inhibited because the stem loop cannot form from this inverted repeat ----- there is another GC-rich sequence complementary to the first repeat in front of the termination signal (sequence 2 in Figure). This sequence takes part in forming an alternative stem loop , which does not lead to premature termination because it is not followed by a strech of adenines. Why does this alternative stem loop not form normally? Because if translation occurs with normal speed, the ribosome will already cover the alternative
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Unformatted text preview: complementary GC-rich sequence and prevent alternative stem loop formation. Under low tryptophan levels translation is slowed at an earlier sequence of the mRNA (sequence 1 in Figure) simply because it contains codons for tryptophan, and tryptophan is scarce for translation. So the level of tryptophan directly determines the speed of translation , which in turn determines the kind of stem loop that is formed , either the one that can prematurely terminate transcription (high tryptophan levels) or the one that cannot (low tryptophan levels)....
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