Vitellogenin as a multigene family. Not all Xenopus vitellogenin genes may be in an "expressible" configuration.

DNA-cDNA titrations suggest that the Xenopus genome comprises two albumin genes, whereas there are 12 to 16 vitellogenin-like genes. Both sets of genes are expressed in liver, those coding for albumin constitutively, but vitellogenin only when induced by estrogen in both male and female frogs. Mild micrococcal nuclease digestion of liver nuclei to separate "expressed" from "unexpressed" gene fractions (Tata, J. R., and Baker, B. S. (1978) J. Mol. Biol. 118, 249-272) revealed that the multiple vitellogenin genes are found in two chromosomal states, here termed as "expressible" and "nonexpressible." Both albumin genes were in the chromosomal state associated with expression, but of the 12 to 16 vitellogenin genes, only 4 to 6 are in the potentially expressible state. This distribution pattern is independent of sex and only slightly modified during induction or de-induction of vitellogenesis by estrogenic stimulation or withdrawal.