Metabolic stability of the two forms of initiation factor IF-3 in Escherichia coli during the growth cycle

Possible alteration in the ratio of the long and short forms of initiation factor IF-3 (FEBS Lett. 79, 264-275, 1977) during the growth cycle of Escherichia coli was examined. The ratio was found to remain unchanged between the exponential and stationary growth phases. Contrary to an earlier report (Eur. J. Biochem. 29, 319-325, 1972), the total amount of IF-3 relative to the ribosome content in stationary phase cells was essentially the same as in midlogarithmic phase cells. The activity of IF-3, assayed after its separation from other initiation factors by chromatography, was also the same in extracts from midlogarithmic and stationary phase cells. The data show that in Escherichia coli the ratio of IF-3/ribosome is maintained constant. The ribosomes themselves have been shown to retain virtually full activity in vitro during this transition indicating that growth-cycle-dependent biochemical modifications of the ribosome do not affect its protein synthetic capacity per se.