BETA-1,4-GALACTOSYLTRANSFERASE AND LACTOSE BIOSYNTHESIS - RECRUITMENTOF A HOUSEKEEPING GENE FROM THE NONMAMMALIAN VERTEBRATE GENE POOL FORA MAMMARY-GLAND SPECIFIC FUNCTION
Nl. Shaper et al., BETA-1,4-GALACTOSYLTRANSFERASE AND LACTOSE BIOSYNTHESIS - RECRUITMENTOF A HOUSEKEEPING GENE FROM THE NONMAMMALIAN VERTEBRATE GENE POOL FORA MAMMARY-GLAND SPECIFIC FUNCTION, Journal of mammary gland biology and neoplasia, 3(3), 1998, pp. 315-324
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.