Genetic control of ornithine transcarbamylase induction in chick kidney

Sex-linkage of the chicken ornithine transcarbamylase gene

The chicken ornithine transcarbamylase gene has been mapped to the short arm of the Z chromosome, near the centromere. Radioactive in situ hybridization was performed using a human cDNA clone, and chromosomes were scored for silver grain accumulation. This represents the first documented case of a mammalian X-linked gene also being sex-linked in an avian species. The significance of this finding in the context of sex chromosome evolution is briefly discussed. Further, differential sex-related OTC expression, whereby females (ZW) consistently produce higher levels of the enzyme than males (ZZ), must imply a peculiar mode of OTC transcription regulation.

Chicken ornithine transcarbamylase: its unexpected expression

Ornithine transcarbamylase (OTC), one of the enzymes of the urea cycle, is detectable in some strains of chickens, although they have no functional urea cycle. The enzyme consists of three identical subunits of 36 kd and is present in mitochondria of the kidney. Using immunoabsorbent column chromatography, we found further evidence that the enzyme is detectable as a precursor form (40 kd) in chicken brain, heart, liver, pancreas, gizzard, small intestine, and breast muscle. When an extract of small intestine containing only precursor OTC was treated with a kidney extract, the precursor was converted into OTC. This suggests that there is a tissue-specific processing protease in the kidney which splits a peptide off the precursor, causing the expression of OTC activity in this organ. However, the reason why the enzyme or its precursor is expressed in these organs is not known. The results of this study suggest that, unlike mammals, chickens are more organ specific with regard to the ability to incorporate precursor OTC into mitochondria.

Phylogenetical and ontogenetical studies on the molecular weight heterogeneity of bovine serum transferrin

Antitransferrin (Tf) rabbit serum was highly specific: it reacted with Tfs of ruminants, such as European breeds and Zebu breeds of cattle, Bali cattle, banteng, swamp and river types of water buffalo, anoa, goat, sheep, deer, antelope, camel, and giraffe, but did not react with serum of other non-ruminant species, such as pig, wild boar, hippopotamus, horse, rabbit, rat, chicken, etc. Electrophoresis of Tf and immunoglobulin G (IgG) complexes was carried out using sodium dodecyl sulfate--polyacrylamide gel electrophoresis (SDS-PAGE). Within ruminants, the following species showed two Tf molecules on SDS-PAGE; European and Zebu cattle, Bali cattle, banteng, two types of water buffalo, and two species of anoa. Other ruminants, sheep, goat, deer, antelope, camel, and giraffe, etc., showed only one Tf molecule. The Tf heterogeneity in molecular weight was, thus, restricted to Bos, Bubalus, and Anoa. The molecular weight of Tf of water buffalo was slightly larger than that of cattle on the gel. The peptide pattern from cyanogen bromide cleavage of Tf of the water buffalo differed clearly from that of cattle. Fetal Tf showed only one molecule during development, but a newborn calf has two Tf molecules, (one large and one small) within 18 hr after birth. We suggest, therefore, that the small molecules formed during the last month of gestation. The peptide patterns of adult and fetal Tfs cleaved by cyanogen bromide differed with regard to the two large peptides; fetal Tf, lacking the second-largest peptide, had twice the amount of the largest peptide compared with adult Tf. From these results, we suggest that a change in peptide sequence occurs from the last month of gestation, when the largest peptide is degraded to the second largest. However, a Tf-like protein detected in the liver microsomal fraction has only one molecular size, both in adult and in fetal livers.

Genetically controlled quantitative variation of ornithine transcarbamylase in the chick kidney

This experiment was made to show that the marked variation in ornithine transcarbamylase (OTC) activity observed within a chicken breed or among breeds is due to quantitative changes, not qualitative ones. The enzyme was partially purified from three different chicken breeds, the White Leghorn B line, the Cochin Bantam breed, and a commercial line named "G," by the following steps: (i) extraction of OTC with Triton X-100 and cetyl-trimethylammonium bromide, (ii) heating, and (iii) salting-out column chromatography. No difference was shown immunologically, enzymologically, or physicochemically among the partially purified OTCs. The enzyme amount determined using anti-bovine OTC antiserum was related linearly to the enzyme activity either from the same chicken breed or from different breeds. These results suggest that marked variation in OTC activity reflects variation in the amount of enzyme synthesized in the kidney, and this is controlled by regulatory genes encoded on an autosome, not the structural gene.