Ribonucleic acid synthesis and transport in animal cells at 27°C

The failure to induce glutamine synthetase in retinal cells with hydrocortisone at 27°C (37°C is the normal temperature for induction) led to a study of some aspects of RNA synthesis at 27°C by using the method of selective labelling with radioactive precursors, sucrose-density-gradient centrifugation and radioautography. The low-temperature treatment not only decreases the rate of RNA synthesis but also interferes with the general pattern of RNA maturation and distribution. It has been found that nuclear–cytoplasmic exchange is severely altered at low temperature and may be an explanation of the failure to induce the enzyme. When the temperature is raised from 27°C to 37°C, the incorporation of exogenous radioactive uridine into the RNA is distorted by a pool-dilution effect that results in an ‘over-shoot’ phenomenon. The kinetics of labelled uridine equilibration into the UMP pool accounts for the difficulty encountered by many investigators in ‘chase’ efforts with unlabelled nucleosides.