Proteins degradation, environmental conditions

ER-Associated Degradation, when proteins mis-fold in the ER due to mutation or environmental conditions, they are selectively exported to the cytosol for degradation by the proteasome. 

In prokaryotes such as E. coli, most of the control of protein synthesis occurs at the level of transcription. (Refer to Section 18.3 for a discussion of the principles of prokaryotic transcriptional control.) This circumstance makes sense for several reasons. First, transcription and translation are directly coupled that is, translation is initiated shortly after transcription begins (Figure 19.8). Second, the lifetime of prokaryotic mRNA is usually relatively short. With half-lives of between 1 and 3 minutes, the types of mRNA produced in a cell can be quickly altered as environmental conditions change. Most mRNA molecules in E. coli are degraded by two exonucleases, referred to as RNase II and polynucleotide phosphorylase. 

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