Nutritional regulation of the synthesis and degradation of malic enzyme messenger RNA in duck liver.

The amount of malic enzyme mRNA in total liver RNA increased rapidly when starved ducklings were fed a high-carbohydrate mash diet, reaching 15 times the initial level at 9 h and an apparent steady state, about 20 times the initial level, at 24 h. Based on the kinetics of accumulation, malic enzyme mRNA had a half-life of 3-5 h in the livers of fed ducklings. When fed ducklings were starved, malic enzyme mRNA decreased with a half-life of about 1 h. Feeding, therefore, may have inhibited the degradation of malic enzyme mRNA, but not sufficiently to account for the 20-fold increase in mRNA level. The level of malic enzyme sequences in nuclear RNA increased severalfold when starved ducklings were fed, consistent with a stimulation of transcription. The rate of transcription of the malic enzyme gene, as measured in isolated nuclei, increased 3-5-fold when starved ducklings were refed. Starvation of fed animals caused a 55-65% inhibition of the transcription of the malic enzyme gene. Synthesis of albumin mRNA was little affected by refeeding or starvation, indicating that the observed effects on synthesis of malic enzyme mRNA were selective. We conclude that both increased transcription and decreased degradation contribute to the 20-fold increase in malic enzyme mRNA caused by feeding starved ducklings.