An age-specific 35-kDa phosphoprotein binds to a repressor element in the ovalbumin gene promoter in the avian species Japanese quail.

The ovalbumin (Ov) gene is expressed in the tubular gland cells of the avian oviduct in a specific manner under the control of developmental, tissue-specific, and hormonal cues. The expression is controlled by an array of positive and negative cis-acting elements present up to 1 kb upstream of its transcription start site. Our findings presented in this communication indicate that a well-characterized repressor element may be involved in active repression of the gene during aging. At least two proteins bind to the 25-bp sequence used in the present study encompassing the COUP adjacent repressor (CAR) element. The binding of one of the trans-acting factor that interacts with the repressor element increases during aging. This is accompanied by a decrease in transcription of the gene. The binding of the factor-to-repressor element decreases when expression of the Ov gene is induced by steroid administration. The factor has an approximate molecular weight of 35 kDa and is a phosphoprotein. It loses its ability to bind to DNA upon dephosphorylation. This makes it a potential target of various kinases/phosphatases that relay the various developmental, tissue-specific, and hormonal cues. The other trans-acting factor is a single-strand specific protein that interacts with the repressor element in an age-independent manner. These two proteins acting in conjunction may be involved in the repression of the Ov gene in old female birds where the lower circulating level of steroid hormones may be acting as an age-related cue.

DNA methylation induced changes in chromatin conformation of the promoter of the vitellogenin II gene of Japanese quail during aging.

One approach to the understanding of the molecular basis of aging in higher organisms may be to use genes whose timing and rate of expression during the life span run parallel with specific functions that can be monitored. The genes for egg proteins, such as vitellogenin (VTG), which is expressed in the liver, and ovalbumin, lysozyme etc. that are expressed in the oviduct of birds, meet these requirements. Egg laying function is dependent on the production of these proteins, which, in turn, depends on the expression of their genes. In this communication we present the age-related studies on the VTG II gene of the bird, Japanese quail. The gene is expressed only in the liver and its expression is considerably lower in old birds that do not lay eggs. Comparison of the promoter region of the gene carrying the two important cis-acting elements, estrogen responsive element (ERE) and progesterone responsive element (PRE), shows it to be 100% homologous to the corresponding region of the chicken VTG II gene. Methylation of DNA and conformation of chromatin of this region were studied, as they are known to be important for regulation of expression of genes. Our studies show that in the liver of adult female quails which lay eggs, a -CCGG- sequence located in this region is hypomethylated, and the chromatin encompassing this region of the gene is relaxed. In the old, the -CCGG- sequence is hypermethylated and the chromatin is compact. This is correlated with a decrease in the expression of the gene and decrease in egg production. Further, electrophoretic mobility shift assay (EMSA) shows that the levels/affinity of specific trans-acting factors that bind to ERE and PRE present in the region, are not different in adult and old birds. Hence the methylation status of the -CCGG- sequence that is located in-between the ERE and the PRE may be crucial for the conformation of chromatin and availability of these two important cis-acting elements for the binding of the trans-acting factors. This, in turn, may downregulate the expression of the gene in old birds.