No abstract

Galactokinase is an essential enzyme for the metabolism of galactose and its deficiency causes congenital cataracts during infancy and presenile cataracts in the adult population. We have cloned the human galactokinase cDNA, which maps to chromosome 17q24, and show that the isolated cDNA expresses galactokinase activity in bacteria and mammalian cells. We also describe two different mutations in this gene in unrelated families with galactokinase deficiency and cataracts. The availability of the cloned galactokinase gene provides an important reference to identify mutations in patients with galactokinase deficiency and cataracts.

Three S-allele-associated proteins (S-proteins) of Petunia inflata, a species with gametophytic self-incompatibility, were previously found to share sequence similarity with two fungal ribonucleases, RNase T2 and RNase Rh. In this study, the S-proteins from P. inflata plants of SS2 and S2S3 genotypes were purified to homogeneity by gel filtration and cation-exchange chromatography, and their enzymatic properties were characterized. The three S-proteins (Si, S2, and S3), with pairwise sequence identity ranging from 73.1 to 80.5%, were similar in most of the enzymatic properties characterized. The ribonuclease activity had a pH optimum of 7.0 and a temperature optimum of 500C. Diethylpyrocarbonate at 1 millimolar almost completely abolished the ribonuclease activity; cupric sulfate and zinc sulfate at 1 millimolar reduced the ribonuclease activity of the three S-proteins by 50 to 75%. EDTA and RNasin had no inhibitory effect. All three Sproteins hydrolyzed polycytidylic acid preferentially, but varied in their nucleolytic activity toward polyadenylic acid and polyuridylic acid.Gametophytic self-incompatibility acts to prevent self-fertilization and consequent inbreeding in many species of the Solanaceae, including Petunia inflata. In Petunia, self-incompatibility is controlled by a single polymorphic locus, the Slocus. The S-allele products of pollen and pistil are presumed to interact in a manner that allows pistils to discriminate between self-and cross-pollen. The result of this specific selfrecognition event is that pollen bearing an S-allele identical to either one of the two alleles carried by the pistil suffers growth arrest, while pollen bearing an allele different from both alleles carried by the pistil is able to grow down the pistil to effect fertilization.S-allele-associated pistil proteins (S-proteins) have been identified in many species with gametophytic self-incompatibility (1, 7, 8, 17, 19-21, 23, 24, 29). The sequences of a number of solanaceous S-proteins have been deduced from the corresponding cDNA sequences (1-3, 10, 20, 30). All the full-length cDNA sequences predict a leader peptide that is not found in the mature protein.

Galactokinase (GK; EC 2.7.1.6) is the first enzyme in the metabolism of galactose. In humans, GK deficiency results in congenital cataracts due to an accumulation of galactitol within the lens. In an attempt to make a galactosemic animal model, we cloned the mouse GK gene (Glk1) and disrupted it by gene targeting. As expected, galactose was very poorly metabolized in GK-deficient mice. In addition, both galactose and galactitol accumulated in tissues of GK-deficient mice. Surprisingly, the GK-deficient animals did not form cataracts even when fed a high galactose diet. However, the introduction of a human aldose reductase transgene into a GK-deficient background resulted in cataract formation within the first postnatal day. This mouse represents the first mouse model for congenital galactosemic cataract.

We identified two S-allele-associated proteins (S-proteins) in a self-compatible cultivar of Petunia hybrida based on their segregation in F1 hybrids between P. hybrida and its self-incompatible relative, Petunia inflata (with S2S2 genotype), and in selfed progeny of P. hybrida. These two S-proteins, designated Sx-protein (24 kDa) and So-protein (31 kDa), are pistil specific, and their expression follows a temporal and spatial pattern similar to that of S-proteins characterized in self-incompatible solanaceous species. Their amino-terminal sequences also share a high degree of similarity with those of solanaceous S-proteins. Selfing of P. hybrida yielded plants with SoSo,SxSo, and SxSx genotypes in an approximately 1:2:1 ratio, indicating that the Sx-and So-alleles, though expressed in the pistil, failed to elicit a self-incompatibility response. The S2-allele of P. inflata is expressed in all the F1 hybrids, rendering them capable of rejecting pollen bearing the S2-allele. The So-allele is not functional in the F1 hybrids, because all the F1 progeny with S2So genotype are self-compatible. However, in F1 hybrids with S2Sx genotype, approximately half are self-incompatible and half are self-compatible, indicating that the function of the Sx-allele depends on the genetic background. These results strongly suggest that the presence of functional S-alleles alone is not sufficient for expression of a self-incompatibility phenotype, and reaffirm the multigenic nature of gametophytic self-incompatibility suggested by earlier genetic studies.