Cloning and expression of a pseudo-ovalbumin gene

Evolutionary conservation of vitellogenin genes

Homologous and heterologous hybridizations in solution were performed between sheared genomic DNA and DNA complementary to vitellogenin mRNA of Xenopus, chicken, and migratory locust. The kinetics of hybridization and the thermal stability of the hybrids formed suggested a high degree of conservation of coding sequences of insect, amphibian, and avian vitellogenin genes. These cDNA probes hybridized to calf thymus DNA to a slight, but significant, extent, and not at all to Micrococcus lysodektikus DNA. DNA complementary to Xenopus albumin mRNA did not cross-hybridize significantly with locust or chicken DNA. Further evidence for the evolutionary conservation of vitellogenin genes was obtained from Southern blot analysis of restriction endonuclease-digested genomic DNA from a variety of vertebrate and invertebrate oviparous animals (Xenopus, chicken, migratory and desert locusts, yellow meal worm, carab moth, and Mediterranean fruitfly). When probed with cloned vitellogenin cDNAs from Xenopus and migratory locust, the DNA of these organisms showed varying degrees of homology of parts of the vitellogenin coding sequences. Southern blot analysis also showed that a part of the sequence specified in the cloned Xenopus vitellogenin cDNA was represented as repetitive DNA in the locust genome. However, cloned locust vitellogenin cDNA hybridized to discrete fragments of the restricted vertebrate DNA. These studies demonstrate a remarkably high degree of conservation of insect, amphibian, and avian vitellogenin genes.

The ovalbumin gene family: hormonal control of X and Y gene transcription and mRNA accumulation

The ovalbumin gene family is composed of three genes, X, Y and ovalbumin, which are expressed in laying hen oviduct. We have analyzed the in vivo transcription products of X and Y genes and the effect of steroid hormones on their synthesis and accumulation. As in the case of ovalbumin, the complete gene transcripts and processing intermediates are present in the poly(A)+ RNA fraction. The mature RNAs are found in polysomes and are translated into proteins. The expression of X and Y genes is controlled by steroid hormones: X and Y RNAs are not detectable in oviducts from chicks withdrawn from estrogen stimulation, whereas in chicks stimulated with estrogen for 7 days, X RNA represents 0.3% and Y RNA 0.8% of ovalbumin mRNA. In laying hen, however, the levels of X and Y RNAs are about 2% of ovalbumin mRNA. After stimulation with other steroid hormones, alone or in combination, the level of X and Y RNA does not achieve that detected in laying hen. Progesterone has a much weaker effect on X RNA accumulation than on that of Y and ovalbumin mRNAs. Studies with isolated nuclei show that X and Y gene expression is regulated by hormones at the level of transcription. However, the differences observed between the transcription rates and the accumulation of X and Y mRNAs suggest that the expression of X and Y genes could also be controlled at the levels of RNA processing and/or mRNA stability.

Differential hormonal responsiveness of the ovalbumin gene and its pseudogenes in the chick oviduct

We describe the isolation of recombinant phages from a chicken gene library which contain two genes designated X and Y. These two genes are linked to the ovalbumin gene (OV) in the order 5'-X-Y-ovalbumin-3' [Royal, A., Garapin, A., Cami, B., Perrin, F., Mandel, J. L., LeMeur, M., Bregegegre, F., Gannon, F, LePennec, J. P., Chambon, P., & Kourilsky, P. (1979) Nature (London) 279, 125-132]. Both genes contain multiple intervening sequences and share limited sequence homology with the authentic ovalbumin gene but are expressed in oviduct cells at different levels. X and Y hybridization probes were prepared in order to study the expression and the relative hormonal responsiveness of these three genes in chicken oviduct. The sequence specificity of the probes was demonstrated by Southern hybridization assays. Northern hybridization studies using the X and Y gene probes indicated the presence of putative precursor molecules in stimulated oviduct ribonucleic acid preparations, which differ in size from those observed for ovalbumin. R0t analysis has demonstrated that, similar to the ovalbumin gene, the level of X and Y gene transcripts is increased by the steroid hormone estrogen, but to varying degrees. The extent of hormonal responsiveness of the three closely related genes is in the order (normalized) relative to ovalbumin of OV:Y:X congruent to 100:10:1. Pulse-labeling studies of these three closely linked genes suggest that in estrogen-stimulated oviduct, the markedly different steady-state levels of the X, Y, and ovalbumin gene transcripts reflect their differential transcription rates.

The ovalbumin gene family: structure of the X gene and evolution of duplicated split genes

The X, Y and ovalbumin genes, which are found within a 40 kb region of the chicken genome, are all expressed in oviduct under steroid hormone control, and share some sequence homologies. We have now cloned the complete X gene and have analyzed its structure. It codes for two RNA species, X and X'; both are coded by eight exons and appear to differ only by the size of their 3' untranslated region, X' RNA being 1400 nucleotides longer than X RNA. The striking similarity in the number and length of the exons which constitute the X, Y or ovalbumin genes establishes that they have evolved from a common ancestor gene by duplication events. Comparison of selected regions of the X and ovalbumin genes indicates that the exon sequences coding for protein and the location of the splice junctions have been well-conserved. The introns and the 3' untranslated exonic sequences have diverged much more rapidly. Four regions of apparently unrelated repetitive sequences are found both outside the X gene and within it (in two introns and in the sequence coding for the 3' untranslated part of X'RNA). The intragenic repetitive sequences have no counterpart in the ovalbumin and Y genes.