Purification of a calcium dependent ribonuclease from Xenopus laevis.

We have purified a Ca2+ dependent ribonuclease from the oocytes of Xenopus leavis. Two properties of this ribonuclease set it apart from other known nucleases. First, Ca2+ was required for ribonuclease activity, and Mg2+ would not substitute. Second, the enzyme specifically degraded RNA and digestion of double or single stranded DNA was not observed. Ca2+ dependent ribonuclease activity of the purified 36-kDa protein was directly observed after renaturation of the protein following electrophoresis in an SDS-Laemmli gel. In addition, the enzyme was shown to have endoribonuclease activity at numerous sites. The Ca2+ dependence suggests that the ribonuclease activity may be modulated by changes in the level of intracellular Ca2+ and thereby provide a direct link to signal transduction systems.

Kinetic control of 5 S RNA gene transcription.

We have determined that the differential transcription of somatic and oocyte-type 5 S RNA genes in a Xenopus laevis oocyte extract is a consequence of vastly different rates of stable complex assembly. Somatic-type 5 S RNA genes sequester a limiting transcription factor much more rapidly than oocyte-type 5 S RNA genes. Once formed, however, transcription complexes on both types of genes are stable, and are transcribed at nearly equivalent rates. The relative rates of stable transcription complex assembly are strongly dependent on the concentration of Mg2+. Kinetic differences in transcription complex assembly provides a key distinguishing feature between these two genes which may be used in the selective repression of oocyte-type 5 S RNA genes during the early development of Xenopus, and may also be utilized in other systems of regulated gene expression.