Lactating mammary gland from various species expresses two major mRNAs for beta1,4galactosyltransferase; one 4 kb and other 2.5 kb. The 4 kb mRNA has a 2.7 kb long 3' noncoding sequence. The 2.5 kb mRNA is short 1.5 kb of the 3' end sequence of 4 kb mRNA and is generated by utilizing the polyadenylation signal present within the 3' noncoding sequence of the 4 kb mRNA. Primer extension analysis of the mRNA from various tissues show that the beta1,4galactosyltransferase mRNA in the lactating mammary glad is about 230 bases shorter at the 5' end compared to the mRNA produced in kidney and embryonal trachea cells. The mammary glad mRNA has only the second in frame ATG codon compared to the mRNA from kidney and embryonal trachea and thus codes for a protein that lacks the first thirteen residues but still contains the putatative membrane-anchoring domain. In the lactating mammary gland in addition to the two major mRNA species other beta1,4galactosyltransferase sequence-related transcripts were identified. These sequence-related transcripts are generated by utilizing the polyadenylation signals within the introns for Poly A+ synthesis and by partial processing of the precursor RNA. Sequence analyses of many of these transcripts showed that none of these Poly A+ containing RNAs have any long open reading frame within the sequences which are not present in the 4 kb or 2.5 kb mRNAs. The significance of very high proportion of these aborted molecules in the lactating mammary gland remains to be determined.
