Monday, November 06, 2006

Opposite to expectations

In past blogs I have alluded to some analyses where I compared sxy mRNA and protein abundance between strains carrying mutations in sxy. I found that hypercompetent sxy mutants produce on average about twice as much transcript but 10-50 times as much protein as wild type cells. In other words, a sxy transcript in a hypercompetent mutant is translated much more efficiently than a transcript in a normal cell. Thus, sxy appears to be post-transcriptionally regulated

I thought these results may be telling us about how sxy expression is regulated in normal cells. I predicted that transferring cells from rich to starvation medium would result in a slight increase of sxy promoter activity and a large increase in translation of sxy. I have crunched the data and, very surprisingly, found the exact opposite result. Upon transfer to MIV, sxy transcript levels immediately skyrocket by 25 fold, then quickly return to pre-induction levels. Protein levels show a much more gradual increase to about 10 fold pre-induction levels, followed by a slow decline.

This result is consistent with what we know about the relative stability of the mRNA and protein – the former is generally very short lived and the latter is long lived. However, it means that cells do not use post-transcriptional regulation to induce sxy upon transfer to inducing conditions – or, if they do, the effect is masked by a very strong inducing signal acting at the sxy promoter. The result may also indicate that in MIV sxy transcript half-life is insufficient to allow significant translation, hence the only moderate increase in protein level. In other words, it is the opposite of what I described above. In a hypercompetent mutant in rich medium, one transcript can make a lot of protein, whereas in wild type cells in MIV, some transcripts may be degraded before even making one complete protein.


At 6:47 PM, Blogger Heather Maughan said...

I don't think it is that surprising. It seems like you would expect exactly what you found but maybe I am missing something.

Doesn't this still tell you that sxy is regulated post-transcriptionally? If the wild type mRNA had a stronger secondary structure then it would hang around longer (as in the hypercompetent mutants) and would have a greater probability of being translated. If it had a weaker one, then it would be degraded more quickly and have a lower probability of being translated. It seems like this is what you would expect. The mutants are hypercompetent because their secondary structure allows them to hang around longer and be translated.


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