Activation of Protein Synthesis by Microsomes from Aging Beet Disks

Microsomal fractions isolated from sterile, aged disks of red beetroot incorporate leucine into protein when supplemented with the supernatant fraction, ATP, GTP, and KCl; the incorporation is sensitive to RNase and is not due to bacteria. The microsomal activity is inhibited by puromycin and cycloheximide but is virtually insensitive to both d-threo and l-threo-chloramphenicol, as predicted from physiological studies.Microsomes isolated from fresh disks have much lower incorporating ability than those from disks aged for 1 or 2 days; maximal activity occurs when the rate of protein synthesis by the intact disks is highest. The low activity of fractions from fresh disks is attributable to a deficiency in the microsomal fraction and not to the supernatant fraction; it is not due to a dissociable inhibitor. The RNA content of the microsomal fraction increases with aging and so the increase in incorporating ability may be due to a synthesis of messenger RNA induced by slicing, rather than to an activation of pre-existing messenger. These results support the view that the aging phenomenon involves a derepression of gene activity.