5 S DNAs of Xenopus laevis and Xenopus mulleri: evolution of a gene family

The 5 S DNA which contains the genes for 5 S RNA has been purified from the frog Xenopus mulleri and compared with the 5 S DNA of Xenopus laevis. Both DNAs contain highly repetitive sequences in which the gene sequence that codes for 5 S RNA alternates with a spacer sequence. The 5 S DNAs of X. laevis and X. mulleri comprise about 0.7% of the total DNA or about 24,000 and 9000 repeating sequences, respectively. The average repeat length within native X. laevis and X. mulleri 5 S DNA is about 0.5 to 0.6 and 1.2 to 1.5 × 10⁶ daltons, respectively, each repeat of which contains a single gene sequence for 5 S RNA (0.08 × 10⁶ daltons). The two DNAs differ in the average length of their spacers and no cross homology can be detected by heterologous hybridization of the two DNAs, except within the 5 S RNA gene regions. Despite their differences, the spacer sequences of X. laevis and X. mulleri 5 S DNA resemble each other enough to conclude that they have diverged from a common ancestral sequence.The multiple repeating sequences of 5 S DNA in each species have evolved as a family of similar, but not identical sequences. It is known that 5 S DNA is located at the ends (telomeres) of the long arms of most, if not all, X. laevis chromosomes. It is proposed that multiple gene sequences located on the ends of many chromosomes can evolve together as a family if there is extensive and unequal exchange of DNA sequences between homologous and non-homologous chromosomes at their ends.