EXPRESSION ANALYSIS OF THE INDIVIDUAL BOVINE BETA-CASEIN, ALPHA(S2)-CASEIN AND KAPPA-CASEIN GENES IN TRANSGENIC MICE

To identify cis-acting regulatory elements involved in the regulation of expression of the casein genes, the bovine beta-, alpha(s2)- and ka ppa-casein genes were isolated from cosmid libraries and introduced in to the murine germline. Bovine casein expression was analysed at the R NA and protein level. The bovine beta-casein gene, including 16 kb of 5 '- and 8 kb of 3 '-flanking region, appeared to be expressed in all 12 transgenic mouse lines analysed. In 50 % of these lines expression levels in milk exceeded 1 mg/ml. Three lines displayed expression leve ls comparable with or well above (20 mg/ml) the beta-casein levels in bovine milk. Transgene expression was restricted to the mammary gland. Strong induction of expression occurred at parturition and thus resem bled the bovine rather than the murine pattern. In spite of this high- level tissue-specific and developmentally regulated expression, beta-c asein expression levels were integration-site-dependent, suggesting th at not all elements involved in regulation of expression were included in this beta-casein clone. Neither the bovine alpha(s2)- nor the kapp a-casein gene, including 8 kb and 5 kb of 5 '- and 1.5 kb and 19 kb of 3 '-flanking sequences respectively, were properly expressed in trans genic mice. However, they were transcribed in stably transfected mouse mammary epithelial cells. This indicates that regulatory elements req uired for high level, mammary gland-specific expression are not presen t in the alpha(s2)-and kappa-casein clones used in this study and are probably located elsewhere in the casein gene locus.