Alternative 5' exons in c-abl mRNA

The cellular abl proto-oncogene encodes a protein-tyrosine kinase and is expressed in many cell types in two or three mRNA size species. Four types of mouse c-abl cDNAs have been cloned from 70Z/3 lymphoid cells that have different 5' sequences encoding predicted N-terminal regions of 20-45 amino acids. One of the four cDNAs has a predicted N-terminal sequence of met-gly-gln in common with the gag N terminus of v-abl. The 5' heterogeneity appears to be generated by alternative addition of 5' exons onto a common set of 3' exons. Alternative splicing occurs at the same site at which bcr sequences join to abl sequences in the Philadelphia chromosome translocation.

Isolation of chicken cellular DNA sequences with homology to the region of viral oncogenes that encodes the tyrosine kinase domain

A library of chicken genomic DNA was screened for sequences that could hybridize to a cloned DNA fragment containing the transforming gene (v-fps) of Fujinami sarcoma virus. In addition to c-fps, two unique chicken cellular DNA sequences were isolated that hybridized weakly to v-fps. These sequences hybridized with many other viral oncogenes encoding tyrosine kinases. Sequence analysis of the region where homology was detected revealed a region that is highly conserved among the tyrosine kinases both at the nucleotide and amino acid levels. Although we were unable to detect expression of either chicken cellular DNA sequence in a variety of avian tissues, the data suggest the existence of additional members of the tyrosine kinase gene family. Screening genomic libraries for sequences that hybridize weakly to functional regions of other genes may prove useful for the isolation and characterization of additional members of other gene families.

Purification of human placental estradiol 17 beta-dehydrogenase study of the steroid-binding site

Human placental estradiol 17 beta-dehydrogenase has been purified by affinity chromatography. The purified enzyme is homogenous by polyacrylamide-gel electrophoresis. To study topography of the steroid-binding site, 16 alpha-bromoacetoxyestradiol 3-methyl ether was synthesized with estriol 3-methyl ether, bromoacetic acid, or [2-3H] bromoacetic acid and dicyclohexylcarbodiimide. The steroid alkylates cysteine, histidine, methionine, and tryptophan under physiologic conditions. Being a substrate of the enzyme, it must bind at the steroid-binding site. The steroid inactivates the enzyme in a time-dependent, irreversible manner. Inactivation of the enzyme by excess 16 alpha-bromoacetoxyestradiol 3-methyl ether follows pseudo--first-order kinetics with t1/2 = 1.5 hours. Amino acid analysis reveals that a histidyl residue is carboxymethylated. Estradiol-17 beta slows inactivation; 2-mercaptoethanol stops it. Previous studies have shown a histidyl residue also present at the catalytic region of the active site of 20 beta-hydroxysteroid dehydrogenase from Streptomyces hydrogenans. It is tempting to consider that these histidyl residues may be an essential component for the dehydrogenation of the steroid substrates.