The Chicken Genome Contains Two Functional Nonallelic β1,4-Galactosyltransferase Genes

Two distinct but related groups of cDNA clones, CKβ4GT-I and CKβ4GT-II, have been isolated by screening a chicken hepatoma cDNA library with a bovine β1,4-galactosyltransferase (β4GT) cDNA clone. CKβ4GT-I is predicted to encode a type II transmembrane glycoprotein of 41 kDa with one consensus site for N -linked glycosylation. CKβ4GT-II is predicted to encode a type II transmembrane glycoprotein of 43 kDa with five potential N -linked glycosylation sites. At the amino acid level, the coding regions of CKβ4GT-I and CKβ4GT-II are 52% identical to each other and 62 and 49% identical, respectively, to bovine β4GT. Despite this divergence in amino acid sequence, high levels of expression of each cDNA in Trichoplusia ni insect cells demonstrate that both CKβ4GT-I and CKβ4GT-II encode an α-lactalbumin-responsive, UDP-galactose: N -acetylglucosamine β4-galactosyltransferase. An analysis of CKβ4GT-I and CKβ4GT-II genomic clones established that the intron positions within the coding region are conserved when compared with each other, and these positions are identical to the mouse and human β4GT genes. Thus CKβ4GT-I and CKβ4GT-II are the result of the duplication of an ancestral gene and subsequent divergence. CKβ4GT-I maps to chicken chromosome Z in a region of conserved synteny with the centromeric region of mouse chromosome 4 and human chromosome 9p, where β4-galactosyltransferase (EC 2.4.1.38 ) had previously been mapped. Consequently, during the evolution of mammals, it is the CKβ4GT-I gene lineage that has been recruited for the biosynthesis of lactose. CKβ4GT-II maps to a region of chicken chromosome 8 that exhibits conserved synteny with human chromosome 1p. An inspection of the current human gene map of expressed sequence tags reveals that there is a gene noted to be highly similar to β4GT located in this syntenic region on human chromosome 1p. Because both the CKβ4GT-I and CKβ4GT-II gene lineages are detectable in mammals, duplication of the ancestral β4-galactosyltransferase gene occurred over 250 million years ago in an ancestral species common to both mammals and birds.