Isolation of two closely related vitel flanking regions, from a Xenopus la (recombinant DNA/restriction endonuclease mapping/gene famil

A gene library of Xenopus laevis was con- structedfrom embryonic DNA partially digested with restriction endonucleases Hae III and Alu I and joined to the phage X Charon 4 cloning vectorwith EcoRIlinkers. Nucleotide se- quences from three of the four related vitellogeningenes have been isolated. Two of the genes (called Al and A2) were isolated in their entirety together with long stretches of flanking se- quences. These two closely related vitellogeningenes have lengths of about 21 and 16 kilobases, but both produce a vitel- logenin mRNA of 6.3 kilobases. In amphibians and other oviparousvertebrates, vitellogenin, the precursor of the yolk proteins, is synthesized in the liver under the control of estrogen. We and others have shown earlier that, in Xenopus laevis, hormonal induction is followed by extensive accumulation of stable vitellogeninmRNA in the hepatocytes (1, 2); in fully induced liver, vitellogeninmRNA accounts for almost 50% of the poly(A)-containing RNA (3, 4). Consequently, 70-90% of the newly synthesized protein is vi- tellogenin (refs. 5 and 6; for reviews see refs. 7 and 8). Cloning experiments involving cDNA derived from purified vitello- genin mRNA have led us to the conclusion that the vitellogeninmRNA population is composed of four related mRNAs of identical length (9, 10). We called the four mRNAs Al, A2, B1, and B2. The A and B groups exhibit a sequence difference of about 20%; the sequence difference between Al and A2 or B1 and B2 mRNA sequences is only about 5%. All four mRNAs code for vitellogeninsof similar molecular weights and are expressed simultaneously upon hormone treatment in indi- vidual animals (10, 11). Hybridization of cloned cDNA to Southern blots of uncloned genomic DNA digests showed that the different RNAs are transcribed from distinct genes (10). The biological implications of this "variant repetition" (12) are not known. We now report on the construction of a X. laevis gene library and on the isolation of the two A genes, including their flanking sequences.