Nutritional regulation and tissue-specific expression of the malic enzyme gene in the chicken. Transcriptional control and chromatin structure.

Refeeding starved chicks causes a 25- to 50-fold increase in the level of malic enzyme mRNA in liver. To define the regulated steps, we measured transcriptional activity of the malic enzyme gene using the nuclear run-on assay and a variety of DNA probes specific to the malic enzyme gene. Refeeding starved chicks stimulated transcription of the malic enzyme gene in liver by 40- to 50-fold. An increased transcription rate was detectable at 1.5 h, was maximal at 3 h, and remained high at 24 h of refeeding. The level of nuclear precursor RNA for malic enzyme assessed by hybridization with intron-specific probes was high in liver of refed birds, and barely detectable in that of starved birds. These results indicate that nutritional regulation of the level of malic enzyme mRNA is transcriptional. Low levels of malic enzyme mRNA in brain, kidney, and heart correlated well with low rates of transcription of the malic enzyme gene in these tissues. In contrast to liver, neither the rate of transcription nor the steady-state level of malic enzyme mRNA was affected by refeeding starved birds. A series of DNase I-hypersensitive sites were located within 4000 base pairs upstream of the transcription start site of the malic enzyme gene in liver. The DNase I-hypersensitive region extending from the start of transcription to 400 base pairs upstream was much more pronounced in the refed state than in the starved state. This change in DNase I hypersensitivity followed the same time course as increased transcription of the malic enzyme gene. This DNase I-hypersensitive region also was present at low intensity in kidney and heart independently of nutritional state. The three constitutive DNase I-hypersensitive sites further upstream were present in liver but not in kidney or heart.