Early T7 gene expression: rates of RNA synthesis and degradation, protein kinase dependent termination of transcription, and efficiency of translation.

Abstract

The mode of transcription of early T7 genes starting from one promotor region and generating a unique polycistronic RNA species suggests the appearance of equimolar amounts of the monocistronic species after RNA processing. Rate measurements revealed, however, a disproportion in the generation of the individual early RNA species. The rate of appearance of the promotor-proximal M gene message (nomenclature see in Table 1) is 4–5x the rate of appearance of all other species. This rate pattern is caused by termination behind the M gene because i) the rate of RNA degradation is fairly similar for most RNA species and ii) termination behind the M gene is released in a T7 mutant lacking protein kinase or in wild type infections in the absence of protein synthesis. Then, the RNA species are produced in equimolar amounts. The rate of degradation is similar for all early RNA species except for the protein kinase message. As measured by two independent methods, the physical halflives of M, POL, 1.1, and LIG (nomenclature see Table 1) message were 7–8 min (30°), while KIN RNA was degraded with a halflife of 4 min. The functional halflives were around 50% of the physical halflives. There is apparently no relationship between size of RNA and halflife, and the data suggest specific signals on each RNA which determine the rate of degradation. The monocistronic RNA species are utilized with different rates in translation. The M gene is not only transcribed more often, it is also translated with highest efficiency. The in vivo translation of the POL gene message occurred with the lowest rate.