Role of Prolactin in the Transcription of β‐Casein and 28‐S Ribosomal Genes in the Rabbit Mammary Gland

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Abstract
Isolated mammary nuclei were incubated in the presence of HgCTP and the neosynthesized RNA was isolated with a SH‐Sepharose column. The concentration of β‐casein mRNA and 28‐S ribosomal RNA in the neosynthesized RNA fractions was evaluated using [3H]DNA probes complementary to β‐casein mRNA and 28‐S rRNA respectively. In the unstimulated pseudopregant rabbit, the transcription of both genes was easily detectable. Injections of prolactin progressively enhanced the transcription rate of both genes and preferentially the β‐casein gene. A comparison between the transcription rates and the accumulation of the corresponding gene products in the cell revealed that there is a good correlation between these two parameters for the 28‐S rRNA gene. By contrast, the acceleration of β‐casein gene transcription by prolactin is unable to account for the simultaneous accumulation of β‐casein mRNA, indicating that prolactin is a potent stabilizer of casein mRNA. Injections of CB154 into lactating rabbits (a drug which suppresses the secretion of prolactin by hypophysis), induced a rapid drop of β‐casein mRNA concentration and a slow decline of β‐casein gene transcription. Simultaneously the drug was responsible for a marked and rapid decrease of 28‐S rRNA gene transcription, while the concentration of the rRNA remained elevated. During weaning the transcription of the β‐casein gene and, to a lower degree, the transcription of the 28‐S rRNA gene proceeded more slowly and this phenomenon was accompanied by a progressive decline of the RNA concentrations.