To explore the possibility that genes might compete for expression, we
have studied transgenic mice producing high levels of the sheep milk
protein, beta-lactoglobulin (BLG), in the mammary gland. Mice carrying
one or more transgene loci expressed BLG in milk at levels ranging fr
om 7 to 33 mg/ml. The effects of BLG synthesis on the levels of endoge
nous milk gene expression were examined. No significant increase in to
tal milk protein concentration was recorded even in mice expressing th
e largest amounts of BLG. Measurement of individual milk proteins show
ed that transgene protein was manufactured at the expense of host prot
ein synthesized in the gland. Whey acidic protein production was more
suppressed than casein production. Suppression of endogenous proteins
was matched by a reduction in the corresponding steady-state mRNA leve
ls; in double-transgenic mice, which expressed the largest amounts of
BLG, beta-casein and whey acidic protein mRNA populations were reduced
to 75 and 56% of control levels respectively. We demonstrate that an
exogenous gene competes effectively for expression with endogenous gen
es, Possible mechanisms of competition are discussed.