The Utp14b-jsd Mutation is Responsible for a Specific Step in 18S Ribosomal RNA Processing in Mouse Spermatogenic Cells.

The murine Utp14b gene is an autosomal retrogene derived from the X-linked Utp14a mammalian homolog of the yeast UTP14 gene, which is involved in 18S rRNA biogenesis. A mutation of Utp14b is responsible for the defect in spermatogenesis in homozygous jsd (juvenile spermatogonial depletion) mutant mice. When these mice are ~5 weeks of age, they have numerous germ cells including spermatocytes, but when they are adults, the germ cells do not develop past the spermatogonial stage although the stem cells are maintained. To test whether this gene was acting by the same mechanism in mammals as in yeast, we first examined the levels of 18S rRNA in spermatocytes isolated from 5-week-old wild-type and jsd mutant mice. Although the levels of 28S rRNA were not affected, the levels of 18S rRNA were reduced in the mutants to 75% of control. The 18S rRNA is derived from a long precursor also containing the 28S and 5.8S rRNA sequences and external and internal transcribed sequences (ETS and ITS) that are removed during processing, in the following sequence: 5'ETS-18S-ITS1-5.8S-ITS2-28S-3'ETS. To determine the defect in the processing of the rRNA, we probed Northern blots of RNA prepared from nuclei of these cells with ETS and ITS sequences. Intermediate bands were observed at 41S (18S-ITS1-5.8S-ITS2-28S), 32S (5.8S-ITS2-28S), 30S (5'ETS-18S-ITS1), and 21S (18S-ITS1). There was no effect of the jsd mutation on the level of the 32S band, confirming UTP14B it was not involved in 28S rRNA processing. In contrast, the 30S band was increased and the 41S and 21S bands were decreased in the jsd mutant mice, demonstrating that only the cleavage between the 5'ETS and the 18S rRNA was dependent on UTP14B. These results prove that the Utp14b retrogene derived from the mammalian homolog of the yeast UTP14 gene, and likely the progenitor X-linked Utp14a gene as well, are also involved in 18S rRNA biogenesis and suggests a mechanism for our surprising observation that spermatogenesis was restored by elevating testicular temperature in jsd mutant mice. Since the rate of protein synthesis in many mammalian cells is markedly reduced by mild hypothermia, as is observed in scrotal testes, male germ cells in jsd mutant testes may require elevation to body temperature to compensate for the reduced protein synthesis due to reduced numbers of complete ribosomes. (Research supported by NIH grant HD-40397)

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