A combination of distal and proximal regions is required for efficient prolactin regulation of transfected rabbit alpha s1-casein chloramphenicol acetyltransferase constructs.

In the rabbit, alpha s1-casein is the major casein secreted in the milk. Transcription of the alpha s1-casein gene is induced by PRL. To define the positions of the cis-sequences involved in the control of rabbit alpha s1-casein gene expression by PRL, chimeric genes containing upstream regions of alpha s1-casein gene linked to the chloramphenicol acetyltransferase gene were cotransfected into Chinese hamster ovary cells with the plasmid expressing the rabbit mammary PRL receptor. It was observed that a distal fragment -3442/-3118 was responsible for a high induction of PRL sensitivity when linked in the 5'-position to a chimeric construct (-391/1774)-chloramphenicol acetyltransferase. A cooperation between distal and proximal regions of the alpha s1-casein gene is responsible for the PRL-dependent enhancer activity of the distal fragment. The mammary gland-specific nuclear factor-like binding sequence found around position -90 in the proximal promoter of the alpha s1-casein gene is involved in this cooperation. The distal fragment was further studied to determine the position of regulatory regions. A -3442/-3385 fragment was sufficient to induce a PRL sensitivity similar to that conferred by the larger -3442/-3118 distal fragment, but multiple interactions are likely to exist between other regulatory regions included in this distal fragment. Four DNA-binding regions (I-IV) have been identified within the reduced -3442/-3385 fragment by footprint experiments using rabbit mammary gland or liver nuclear extracts (NE). Protected area III is observed using both NE. Protected areas I, II, and IV are specific for lactating mammary gland NE. The sequences of areas I and IV share several homologies with the sequence of the mammary gland-specific nuclear factor-binding site.