Changes of tissue-specific transcription factors in the rabbit mammary gland during pregnancy and lactation.

In the mammary gland four tissue-specific transcription factors have been found involved in the regulation of milk protein genes: mammary gland-specific nuclear factor (MGF), milk protein binding factor (MPBF), pregnancy-specific mammary nuclear factor (PMF), mammary cell-activating factor (MAF). Rabbit beta-casein gene promoter contains motifs highly homologous (89-100%) to MGF, MAF and PMF consensus sequences. These transcription factors were analysed simultaneously using electrophoretic mobility shift assays (EMSA) on nuclear protein extracts derived from pregnant or lactating rabbit mammary glands. The specificity of these complexes was analysed in cross-competition EMSA experiments. It was shown that in the rabbit mammary gland the DNA-binding activities of MGF, MAF and PMF change during pregnancy and lactation. Protein extracts of nuclei isolated from mammary glands of 15-day pregnant rabbits form fast-migrating low-specific complexes with MGF, MAF and PMF oligonucleotide probes; in the extracts from mammary glands of late pregnant (day 25) and lactating (day 5) rabbits additional slowly-migrating highly specific DNA-protein complexes are formed. Their appearance changes in parallel with the activation of beta-casein gene expression as measured by run-on gene transcription and beta-casein mRNA accumulation.