Nucleotide Sequence of theRing3Gene in the Class II Region of the Mouse MHC and Its Abundant Expression in Testicular Germ Cells

Abstract

TheRING3(NAT) gene is the first and only locus with no obvious function associated with the immune system in the class II region of the human major histocompatibility complex. This gene is a homologue of theDrosophilahomeotic genefemale sterile homeotic(fsh) and encodes a nuclear serine–threonine kinase. To study more about the physiological function of theRING3gene, we isolated a mouse homologue from a genomic library, determined its gene structure, and investigated its expression profile. The mouseRing3gene spans approximately 8 kb and consists of 12 exons encoding a 798-amino-acid protein, sharing as high as 96% amino acid identity with the humanRING3protein. Northern hybridization revealed that theRing3gene abundantly produced 3.8- and 3.0-kb transcripts in the testis but was weakly expressed with 4.6- and 3.8-kb transcripts in somatic tissues. It appears that testis-specific 3.0-kb transcript gives rise to a smaller size Ring3 protein resulting from the usage of the second ATG codon for translational initiation compared to the almost ubiquitous 4.6-kb transcript. In RNAs isolated from fractionated testicular germ cells, the two testicular mRNAs were detected exclusively in the fractions containing a large population of round spermatids and pachytene spermatocytes. Furthermore,in situhybridization on cross sections of seminiferous tubules in the testis showed that the expression of theRing3gene was initiated at the pachytene spermatocyte stage during meiosis and persisted throughout the round spermatid stage during spermiogenesis. These results suggest that theRing3gene plays an important role in spermatogenesis.