The rate of RNA chain growth was measured in vivo in Escherichia coli cultures growing in various media at 29 and 37 °C. For this purpose, the bacteria were allowed to assimilate [3H]uracil or [3H]guanine into their RNA for short time-periods. The RNA was then extracted and hydrolyzed with alkali, and the radioactivity measured in the resulting nucleotides and nucleosides. The data thus obtained allowed the calculation of individual nucleotide step-times in RNA chain growth, the step-time for a particular nucleotide being defined as the average time required for adding the next nucleotide to the end of a nascent RNA chain carrying that particular nucleotide as its growing end. In bacteria growing exponentially in a glucose-Casaminoacids medium at 29 °C with 1.07 cell generations per hour, the uridylic acid step-time was estimated to be 42 msec, the cytidylic acid step-time to be at most 67 msec and the guanylic acid step-time to be 20 msec. Assuming an equality of the step-times of guanylic and adenylic acids, the average RNA chain growth rate was estimated to be 26 nucleotides/second at 29 °C. At 37.5 °C both the generation period as well as the nucleotide step-times were found to be 60% of the corresponding values at 29 °C, giving an average RNA chain growth rate of 43 nucleotides/second. The nucleotide step-times in E. coli growing at 29 °C in a succinate medium with 0.63 cell generation per hour and in a proline medium with 0.33 cell generation per hour were found to be longer than the corresponding step-times during the faster growth in glucose-Casaminoacids medium. The increases in nucleotide step-times with increases in the generation period observed here are not nearly great enough, however, to account for the great variations in the rate of RNA synthesis observed under these three different physiological conditions. Thus bacteria appear to adjust their specific content of RNA according to their physiological needs by varying both the rate of RNA chain growth and the number of nascent RNA molecules under synthesis at any time. © 1969.
