Murine β1,4-galactosyltransferase: Both the amounts and structure of the mRNA are regulated during spermatogenesis

Previously we have shown that the gene encoding murine β1,4-galactosyltransferase (β1,4-GT; UDPgalactose:N-acetyl-D-glucosaminyl-glycopeptide 4-β-D-galactosyltransferase, EC 2.4.1.38) is unusual in that it specifies two sets of mRNAs of about 3.9 and 4.1 kilobases (kb). Translation of the 3.9- and 4.1-kb mRNAs results in the predicted synthesis of two related membrane-bound forms of the protein of 386 amino acids (short form) and 399 amino acids (long form), respectively. In this study we have examined the expression of β1,4-GT during murine spermatogenesis. Spermatogonia contain a 4.1-kb transcript that is comparable in size to the β1,4-GT mRNA identified in somatic cells. During differentiation from spermatogonia (2n) to pachytene spermatocytes (4n), the amount of β1,4-GT mRNA is reduced to barely detectable levels. Continued differentiation to round spermatids (n) is coincident with a renewed production of β1,4-GT mRNA to levels comparable with those detected in spermatogonia. However, the characteristic 4.1-kb mRNA detected in spermatogonia is replaced by two truncated transcripts of 2.9 and 3.1 kb. By S1 nuclease analysis, the 2.9- and 3.1-kb transcripts were shown to encode the same open reading frame as the 4.1-kb transcript found in somatic cells. The shorter round spermatid transcripts arise as a consequence of the use of alternative poly(A) signals. Lastly, we show that, in direct contrast to all somatic tissues and cell lines examined to date, male germ cells synthesize only the long form of the β1,4-GT polypeptide.