HIGH-LEVEL EXPRESSION OF BOVINE BETA-LACTOGLOBULIN GENE IN TRANSGENICMICE

To study the expression of the bovine beta-lactoglobulin (BLG) gene we isolated the BLG gene from a genomic library and introduced it into m urine germline. Bovine BLG gene including 2.8 kbp of 5' and 1.9 kbp of 3' flanking region was expressed efficiently and mammary gland-specif ically in transgenic mice. Expression levels of BLG in milk exceeded 1 mg ml(-1) in all four mouse lines analyzed. However, in two mouse lin es originating from female founders BLG expression levels varied from less than 0.02 mg ml(-1) up to 1 mg ml(-1). In both lines originating from male founders all analyzed female mice excreted bovine BLG into t heir milk at a high and constant level of 1-2 mg ml(-1) BLG expression was stable within individual mice in two successive lactations and th e amount of BLG in the milk of mice correlated with the level of BLG m RNA in the mammary tissue. Methylation analyses of HpaII sites reveale d that transgene copies were on average more methylated in mice which excreted low levels of BLG into their milk. Each mouse line had its ow n methylation pattern and, in addition, each mouse had more or less id entical methylation patterns in mammary gland, brain and kidney DNA. G enomic sequencing of the BLG gene indicated that the promoter region ( bases -162 to +391 with respect to the transcription start site) was h eavily methylated except for distinct CpG sites that were only partial ly methylated both in transgenic mice and lactating cattle. (C) 1998 E lsevier Science B.V. All rights reserved.