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.