DDX3X, the X homologue of AZFa gene DDX3Y, expresses a complex pattern of transcript variants only in the male germ line

Abstract

DDX3X, the functionalXhomologue of the major AZFa gene, DDX3Y, belongs to the highly conserved PL10-subfamily of DEADboxRNAhelicase genes which are functionally conserved fromyeast to man. They are mainly involved in cell cycle control and translation initiation control of gene transcripts with long 5′UTRextensions containing complex secondary structures. Interestingly, in humans both gene copieswere found to be expressed at different phases of human spermatogenesis. Whereas DDX3Y transcripts are translated only in premeiotic male germ cells, theDDX3Xprotein is expressed only in postmeiotic spermatids. In this study,we found that the major class of DDX3Xtranscripts in human testis become activated first after meiosis and at a specific core promoter not active in somatic tissues and not present upstream of the DDX3Y homologue. Two alternative 5′UTR transcript lengths are subsequently produced by an additional testis-specific 5′UTR splicing event. Both transcripts are mainly processed for polyadenylation in their proximal 3′UTR.Aminor transcript class starting at the same male germ line-specific core promoter produces primary transcripts with an extremely long 3′UTR(~17 kb), which is subsequently spliced at distinct sites resulting in six short 3′UTR splice variants (I-VI). Comparative analyses of the DDX3X transcripts in mouse and primates revealed that this complex pattern of male germ line-specific transcript variants first evolved in primates. Our data thus suggest complex translational control mechanism(s) for the human DDX3X gene locus functioning only in the male germ line and resulting in expression of its protein only in the postmeiotic spermatids.