EVIDENCE FOR REDUCED BREAKDOWN OF MESSENGER RNA DURING BLOCKED TRANSCRIPTION OR TRANSLATION IN ESCHERICHIA COLI

Quantitative estimates of polysomes in Escherichia coli starved for uracil have been obtained. Polysomes were found to break down in a diphasic exponential fashion. Approximately half of the polysomes show a half-life of three to four minutes at 37 °C whereas the remainder has a half-life of about 40 minutes. Incorporation of radioactive amino acids was followed during the starvation, and it was found that the semistable fraction of polysomes produces protein at a rate not higher than 5 to 10% of the average exponential value. Inhibition of protein synthesis by omitting three required amino acids from the medium or by adding chloramphenicol protects the polysomes from breakdown. Measurements of the UTP pool were performed during starvation for uracil with or without concomitant inhibition of protein synthesis. In all cases a rapid depletion of the UTP pool was observed. Our interpretation of these data is: one-half of the messenger RNA in exponentially growing cells is under transcription, the other half is finished and detached from the DNA template. The former is protected against breakdown by attachment of the 3′ end to the RNA polymerase. Protection of the free mRNA is observed when translation is blocked and the mRNA remains attached to ribosomes in polysomes. © 1969.