Abstract We have studied the exact sequence requirement for the formation of 3′ termini of the sea urchin H2A mRNA in frog oocyte injection experiments. Point mutations destroying the symmetry of the inverted DNA repeat in the mRNA trailer coding sequences prevent the generation of genuine 3′ termini. Mutants containing complementary base changes are pseudorevertants and allow the production of H2A mRNA with faithful 3′ termini at wild-type levels. Our transcription analyses show that it is primarily the sequence of the transcribed strand that decides whether or not true 3′ mRNA termini are produced. This is evidence that an RNA stem-loop structure, rather than a DNA cruciform, is essential for this process. Spacer sequences are absolutely required, because in their absence only H2A mRNA with spacer transcript extensions are found. Once the canonical CAAGAAAGA and flanking sequences are linked to the H2A gene, H2A messenger is synthesized at a suboptimal rate, which can be increased to wild-type levels by the addition of 80 bp of the spacer immediately adjacent to the H2A gene.