Mettl1-dependent m7G tRNA modification is essential for maintaining spermatogenesis and fertility in Drosophila melanogaster.

Modification of guanosine to N7-methylguanosine (m7G) in the variable loop region of tRNA is catalyzed by the METTL1/WDR4 heterodimer and stabilizes target tRNA. Here, we reveal essential functions of Mettl1 in Drosophila fertility. Knockout of Mettl1 (Mettl1-KO) causes no major effect on the development of non-gonadal tissues, but abolishes the production of elongated spermatids and mature sperm, which is fully rescued by expression of a Mettl1-transgene, but not a catalytic-dead Mettl1 transgene. This demonstrates that Mettl1-dependent m7G is required for spermatogenesis. Mettl1-KO results in a loss of m7G modification on a subset of tRNAs and decreased tRNA abundance. Ribosome profiling shows that Mettl1-KO led to ribosomes stalling at codons decoded by tRNAs that were reduced in abundance. Mettl1-KO also significantly reduces the translation efficiency of genes involved in elongated spermatid formation and sperm stability. Germ cell-specific expression of Mettl1 rescues disrupted m7G tRNA modification and tRNA abundance in Mettl1-KO testes but not in non-gonadal tissues. Ribosome stalling is much less detectable in non-gonadal tissues than in Mettl1-KO testes. These findings reveal a developmental role for m7G tRNA modification and indicate that m7G modification-dependent tRNA abundance differs among tissues.