BETA-1,4-GALACTOSYLTRANSFERASE AND LACTOSE BIOSYNTHESIS - RECRUITMENTOF A HOUSEKEEPING GENE FROM THE NONMAMMALIAN VERTEBRATE GENE POOL FORA MAMMARY-GLAND SPECIFIC FUNCTION

beta 1,4-galactosyltransferase (beta 4GalT-I) is a constitutively expr essed trans-Golgi enzyme, widely distributed in vertebrates, which syn thesizes the beta 4-N-acetyllactosamine structure commonly found in gl ycoconjugates. In mammals beta 4GalT-I has been recruited for a second biosynthetic function, the production of lactose; this function takes place exclusively in the lactating mammary gland. In preparation for lactose biosynthesis, beta 4GalT-I enzyme levels are increased signifi cantly. We show that mammals have evolved a two-step mechanism to achi eve this increase. In step one there is a switch to the use of a secon d transcriptional start site, regulated by a stronger, mammary gland-r estricted promoter. The transcript produced is distinguished from its housekeeping counterpart by the absence of similar to 180 nt of 5'-unt ranslated sequence. In step two, this truncated transcript is translat ed more efficiently, relative to the major transcript expressed in all other somatic tissues.