Expression of protein synthesis initiation factor IF‐3 in vivo was studied by measuring its level in exponentially growing cells as a function of gene dosage. A strain haploid for infC, the gene for IF‐3, was modified to carry one or two additional infC genes giving diploid and triploid strains. Polyploid strains were achieved by the presence of multicopy plasmids expressing the iqfC gene. When IF‐3 levels were measured by quantitative immunoblotting they were found to be proportional to the gene dosage; the presence of a multicopy plasmid thus causes considerable overproduction of IF‐3, enabling large quantities to be purified. When lysates were prepared from freshly grown cells, only IF‐3α(the long form) was detected; however when IF‐3 was purified from a strain containing a multicopy plasmid which overproduced it, the major product found was IF‐3D (the short form, lacking six amino acids from the N terminus). The synthesis of the two IF‐3 forms was also studied by using a cell‐free coupled transcription‐translation system dependent on exogenous DNA: the IF‐3 gene was found to be very efficiently expressed. IF‐3α increased more rapidly than IF‐3β but following the cessation of protein synthesis IF‐3α decreased while IF‐3β still increased.The results suggest that IF‐3β is slowly degraded to the β form. Addition of non‐radioactive IF‐3α, up to fivefold molar excess over ribosomes, to the synthesizing system in vitro did not inhibit IF‐3 synthesis. Synthesis of IF‐3 in vitro appears to be sensitive to guanosine 3′‐diphosphate 5′‐diphosphate.

LESTIENNE, P., MAYAUX, J., BLANQUET, S., DONDON, J., PLUMBRIDGE, J. A., SPRINGER, M., GRUNBERG‐MANAGO, M., HOWE, J. G., & HERSHEY, J. W. B. (1982). Expression of the Gene for Escherichia coli Initiation Factor IF‐3 in vivo and in vitro. European Journal of Biochemistry, 123(3), 483–488. Portico.