Introns in histone genes alter the distribution of 3′ ends

Chimeric genes were constructed which contained either a histone or globin promoter, a human α-globin coding region as a cONA or containing one or both Intervening sequences, and the 3′ end of a mouse histone H2a gene. The genes were introduced into mouse L cells or Chinese Hamster Ovary cells. The genes containing at least one intervening sequence produced two mRNAs in about equal amounts, one which ended at a cryptic polyadenylation site 33 nucleotides 3′ to the normal histone mRNA 3′ end and one which ended at the normal histone 3′ end. In contrast, the same construct containing a globin cDNA yielded mRNA ending only at the correct histone 3′ end. Similar proportions of polyadenylated and nonpolyadenylated mRNA were obtained when the cryptic polyadenylation signal was replaced with the globin polyadenylation signal. More than 90% of the transcripts were accurately spliced. All of the unspliced transcripts had histone 3′ ends.