REGULATION OF BETA-ACTIN AND GAMMA-ACTIN MESSENGER-RNA BY THYROID-HORMONE IN THE DEVELOPING RAT-BRAIN

THE relative sensitivity of the two isotypes of actin mRNA (beta and g amma) to thyroid hormone (T-3) was examined by comparing the steady st ate levels of these mRNAs with their rates of transcription in cerebra from normal and hypothyroid rats of different ages, covering the peri od of synaptogenesis (days 1-20 postnatal). Hypothyroidism was associa ted with a reduction in the steady state level of beta-actin mRNA duri ng the entire period, but levels of gamma-actin mRNA were reduced only The rats of transcription of both beta- and gamma-actin mRNAs were st imulated by T-3 during the early phase but that of beta-actin mRNAs wa s relatively more sensitive to T-3 than gamma-actin mRNA. The lack of correlation between the rates of transcription of beta- and gamma-acti n mRNA at different ages with their corresponding steady-state levels suggests additional post-transcriptional regulation.