The H1A histone variant is an in vivo repressor of oocyte-type 5S gene transcription in Xenopus laevis embryos.

Previous in vitro transcription studies have pointed to the importance of histone H1 for repression of oocyte-type 5S genes of Xenopus laevis. It has been previously reported that in development up to the early gastrula stage, Xenopus embryos contain a large pool of the maternal histone H1 variant H1M but are virtually devoid of histone H1A, H1B, and H1C proteins. At the early gastrula stage, there is an increase in H1A protein synthesis and H1A becomes the predominant H1 histone variant. Concomitant with the significant appearance of H1A protein in chromatin, oocyte 5S transcription is repressed. Here it is shown that there appears to be a direct link between H1A accumulation and inhibition of oocyte-type 5S RNA synthesis. Inhibition of H1A synthesis by a ribozyme targeted to H1A mRNA leads to the continued expression of oocyte 5S genes. H1A is proposed to inhibit major oocyte 5S gene transcription by sealing the nucleosome that is positioned over the major oocyte 5S coding sequences and by driving major oocyte 5S gene chromatin into a higher-order structure in which histone H1A molecules interact cooperatively.