A positive transcription factor controls the differential expression of two 5S RNA genes.

Somatic 5S RNA genes are transcribed preferentially over oocyte 5S RNA genes by 25- to greater than 200-fold when mixtures of cloned genes are injected into cleaving Xenopus embryos. This preference is an order of magnitude greater than that observed in cell-free extracts. Mutations that decrease the binding of the 5S RNA gene-specific transcription factor TFIIIA to the 5S RNA genes' internal control regions are exaggerated when assayed by embryo injection compared to in vitro transcription. Injection of TFIIIA into cleaving embryos greatly increases endogenous oocyte 5S RNA synthesis at midblastula even when DNA replication is inhibited. Much, but not all, of the preference for somatic over oocyte 5S RNA gene transcription in somatic cells can thus be attributed to the concentration of TFIIIA and to differences in binding constants of TFIIIA to the internal control regions of the two types of genes.