Processing and nucleo-cytoplasmic transport of histone gene transcripts. Precursors of Xenopus and sea urchin histone mRNAs were synthesized in vitro with the SP6 transcription system, and their maturation and nucleo-cytoplasmic transport was studied by frog oocyte injection. 3'' processing is most rapid for homologous histone messenger sequences and does not require either genuine 5'' or specific 3'' ends of the precursor, but capping of the 5'' terminus strongly influences the efficiency of 3'' processing. No generation of 5'' histone mRNA ends can be detected when precursors containing 5'' spacer sequence extensions are injected into the oocyte nucleus. This finding may have some implications for the question whether histone gene transcription could be polycistronic. Using a novel oocyte technique, we have separated nuclei from cytoplasm and have studied the time course of exit of the processed RNA from the oocyte nucleus into the cytoplasm. The results suggest that RNA maturation and nucleo-cytoplasmic transport are not temporally coupled processes.