The chicken genome contains two functional nonallelic beta 1,4-galactosyltransferase genes - Chromosomal assignment to syntenic regions tracks fate of the two gene lineages in the human genome

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