Kinectics of beta-galactosidase synthesis in Escherichia coli at 5 C

Abstract

The defect in protein synthesis that is observed in Escherichia coli after transfer to low temperature was studied. For the enzyme beta-galactosidase, the elongation reactions of transcription and translation can take place slowly but normally at 5 C. The time necessary to complete the coupled synthesis of the beta-galactosidase messenger ribonucleic acid and polypeptide chain was found to be about 80 min at 5 C. From this result and from the known length of the beta-galactosidase monomer, it is possible to calculate that at 5 C one amino acid is added to the growing polypeptide chain every 4 s. The initiation of transcription of the beta-galactosidase messenger is inhibited after transfer to 5 C. This fact alone, however, cannot account for all of the phenomena observed at 5 C, because a given amount of messenger yields less enzyme at 5 C than it does at 37 C. Furthermore, in cells induced for short periods at 37 C, the capacity to synthesize beta-galactosidase after transfer to 5 C was found to accumulate linearily with the square of the time of induction. Two alternative models could account for these data. If all ribosomes that initiate translation at 37 C yield complete beta-galactosidase polypeptide chains at 5 C, then an inhibition of translation initiation after transfer to 5 C must be invoked to explain the results. If, on the other hand, a substantial portion of the ribosomes that initiate translation at 37 C do not yield complete beta-galactosidase polypeptides at 5 C, then intracistronic polarity could account for the data, and there is no need to invoke an inhibition of translation initiation at 5 C.