MURINE BETA-1,4-GALACTOSYLTRANSFERASE - ROUND SPERMATID TRANSCRIPTS ARE CHARACTERIZED BY AN EXTENDED 5'-UNTRANSLATED REGION

We have previously shown that the expression of the gene encoding murine beta-1,4-galactosyltransferase (beta-1,4-GT, UDP-galactose:N-acetyl-D-glucosaminyl-glycopeptide 4-beta-D galactosyltransferase, EC 2.4.1.38) is fundamentally different between somatic and male germ cells (Shaper et al., 1990b). In somatic cells, two transcripts of 3.9 kb and 4.1 kb are produced. In contrast, in spermatogonia only the 4.1 kb transcript is expressed. Maturation of spermatogonia to pachytene spermatocytes is accompanied by reduced expression of the 4.1 kb transcript to barely detectable levels. Continued differentiation to haploid round spermatids is coincident with renewed expression in which the 4.1 kb transcript is replaced by two truncated transcripts of 2.9 and 3.1 kb. In this study, we report the characterization of a full-length beta-1,4-GT cDNA clone from a murine round spermatid library that corresponds to the 2.9 kb transcript. This transcript encodes the same open reading frame as the 4.1 kb transcript, but utilizes alternative poly(A) signals embedded within the long 3'-untranslated region of the somatic transcript. Based on sequence analysis, together with primer extension and Sl nuclease protection experiments, both the 2.9 and the 3.1 kb round spermatid beta-1,4-GT transcripts are distinguished by the presence of an additional 5'-untranslated sequence of approximately 560 bp that is absent in premeiotic germ cells and somatic cells. To understand how a single gene locus can give rise to this haploid male germ cell-specific transcript, we have isolated murine genomic clones and determined that this additional sequence is immediately upstream and contiguous with the transcriptional start site defined for the 4.1 kb somatic cell transcript. These data suggest that a haploid germ cell-specific promoter regulates expression of the beta-1,4-GT gene in murine round spermatids.