AN ALTERNATIVE PATHWAY OF HISTONE MESSENGER-RNA 3' END FORMATION IN MOUSE ROUND SPERMATIDS

During mammalian spermiogenesis, the post-meiotic stage of spermatogen esis, histones are replaced by protamines on the DNA. Despite this his tone elimination, novel polyadenylated histone transcripts were detect ed in mouse round spermatids. Sequence analysis of a spermatid-specifi c H2a cDNA clone indicated that it was derived from a mRNA of a replic ation-dependent histone gene even though its transcript was not polyad enylated in somatic and earlier spermatogenic cells. In round spermati ds, both the hairpin and purine-rich elements in the 3' untranslated r egion of the somatic pre-mRNA were retained in the mature poly(A)(+) m RNA transcripts. Polyadenylation occurred downstream of the purine-ric h element and was not preceded by the somatic AATAAA polyadenylation s ignal sequence. While polyadenylated histone transcripts from replicat ion-dependent genes have been observed previously in somatic cells, ch aracteristics of this type of 3'-end formation in mammalian round sper matids were unique. In particular, a specific replication-dependent H2 a gene was transcribed either as a polyadenylated or nonpolyadenylated transcript in these cells, suggesting that the type of transcript pre sent was dependent on the RNA sequence. Finally, both poly(A)- and pol y(A)(+) mRNAs were found on polyribosomes from round spermatids, indic ating that histones were being translated in these cells and that the polyadenylation status of these transcripts did not affect their trans latability.