Transcriptional fidelity of histone genes injected into Xenopus oocyte nuclei

Abstract

Previous work has indicated that at least some of the genetic information required for the expression of sea urchin histone genes is recognized following injection of the gene repeat (h22) into Xenopus oocyte nuclei^1,2. The ability to elicit the expression of cloned genes and their sequence-manipulated counterparts is proving invaluable in analysing the molecular details of gene expression^2,3. Direct injection of such genes into Xenopus oocyte nuclei remains one of the simplest methods for obtaining such expression and a remarkable degree of transcriptional fidelity has been demonstrated using this system with RNA polymerase III genes^4–9, and to a lesser extent with rDNAs transcribed by RNA polymerase I^10,11. In the case of polymerase II genes there is ample evidence for coupled transcription–translation^12–14, but the degree of transcriptional fidelity involved may, as has recently been shown for the ovalbumin gene¹⁵, be minimal. However, clearly if the oocyte is to be used to investigate transcriptional regulation of such genes, transcriptional fidelity defined as the production of correct RNA termini, rather than the production of ‘functional mRNAs’ (ref. 15), must pertain16. Here we demonstrate such fidelity in the expression of all five Psammechinus miliaris histone genes comprising a repeat unit. However, we find large quantitative variations in the levels of synthesis of the individual correct termini and hence of the mRNAs. In addition to the mRNAs, species with no detectable counterparts in the sea urchin are generated off the coding strand, as are heterogeneous noncoding species.