The ovalbumin gene family: The 5′ end region of the X and Y genes

A cluster of three genes, the X, Y and ovalbumin genes, which are expressed in the oviduct under steroid hormone control, composes the ovalbumin gene family in the chicken. We have now exactly mapped the 5′ end of X and Y precursor RNAs on the corresponding gene sequences and have obtained extensive sequence information on the regions located upstream from the transcription initiation sites. Conservation of the “TATA box” sequences and of the leader exon sequences in all three genes indicates that these elements were included in the gene unit that was duplicated to give rise to this gene family. However, contrary to the ovalbumin and Y leader exons, the corresponding exon in the X gene contains an initiation codon, in phase with the rest of the X protein-coding sequence. Sequence homology between the three genes does not extend more than 80 nucleotides upstream from the transcription initiation sites. A peculiar 300-nucleotide long T+C-rich sequence element has been found in the Y gene 5′-flanking region which has no counterpart in the two other genes. An 11 base-pair repeat of the X gene TATA box sequence, located 760 base-pairs upstream from the X RNA start site, appears to function as a weak promoter in the oviduct of laying hens.