Differential effect of actinomycin D on amino acid incorporation by microsomal components of the housefly Musca domestica

Abstract

Sucrose density gradient centrifugation was used to fractionate the microsomes from the housefly Musca domestica into two components, the free ribosomes and the membranes. The effect of actinomycin D (AM) on the in vivo incorporation of leucine into these fractions was studied. Actinomycin D rapidly inhibited RNA synthesis which however, did not result in a rapid inhibition of incorporation of leucine into the free ribosomes. The half-decay time of this inhibition was about 5 h suggesting a relatively stable m RNA. Under identical conditions the incorporation of leucine into the proteins of the membranes was stimulated about two-fold within 2 h after treatment. This enhanced rate of incorporation could be maintained for at least 4 h by a subsequent injection of actinomycin otherwise it rapidly fell off to normal levels. Puromycin completely prevented this stimulation by actinomycin. It is suggested that this enhanced incorporation is due to de-repression of membrane activity as a result of preferential inhibition of synthesis of mRNA for the repressor protein.