Is there a correction mechanism in the 5S multigene system?

Abstract

RIBOSOMAL GENES are present in many copies per cell in all living species. They are the classic example of the tandemly repeated multigene system, and have been studied in a wide variety of organisms, including man^1,2, Drosophila^3,4, Xenopus^5,6 and Escherichia coli⁷. Reiteration of these genes is presumably necessary to ensure the production of the large amounts of ribosomal and 5S RNA required for use in the ribosomes. However, the possession of multiple copies of a gene poses a special problem for evolution. If a lethal or deleterious mutation occurred in one gene of, for example, the 24,000 present for the 5S DNA in Xenopus⁵, this would have no selective disadvantage for the organism. What then prevents the accumulation of lethal mutations in such a system? Presumably if the number of effective genes were reduced significantly natural selection would operate. But it has been suggested that a ‘correction mechanism’⁸ exists to maintain the homogeneity of the multigene system without a requirement for natural selection. We show here that two cloned repeats of Xenopus oocyte 5S DNA are very similar but not identical in sequence. The similarity strongly suggests that a correction mechanism is operating, but it is not so precise as to cause the different repeats to be identical. A small but distinct degree of heterogeneity is tolerated.