Transcriptional regulation of the human CAD gene during myeloid differentiation

Analysis of the Zebrafish perplexed mutation reveals tissue-specific roles for de novo pyrimidine synthesis during development

The zebrafish perplexed mutation disrupts cell proliferation and differentiation during retinal development. In addition, growth and morphogenesis of the tectum, jaw, and pectoral fins are also affected. Positional cloning was used to identify a mutation in the carbamoyl-phosphate synthetase2-aspartate transcarbamylase-dihydroorotase (cad) gene as possibly causative of the perplexed mutation and this was confirmed by gene knockdown and pyrimidine rescue experiments. CAD is required for de novo biosynthesis of pyrimidines that are required for DNA, RNA, and UDP-dependent protein glycosylation. Developmental studies of several vertebrate species showed high levels of cad expression in tissues where mutant phenotypes were observed. Confocal time-lapse analysis of perplexed retinal cells in vivo showed a near doubling of the cell cycle period length. We also compared the perplexed mutation with mutations that affect either DNA synthesis or UDP-dependent protein glycosylation. Cumulatively, our results suggest an essential role for CAD in facilitating proliferation and differentiation events in a tissue-specific manner during vertebrate development. Both de novo DNA synthesis and UDP-dependent protein glycosylation are important for the perplexed phenotypes.

Myc versus USF: discrimination at the cad gene is determined by core promoter elements

Carbamoyl-phosphate synthase/aspartate carbamoyltransferase/dihydroorotase, which is encoded by the cad gene, is required for the first three rate-limiting steps of de novo pyrimidine biosynthesis. It has been previously demonstrated that cad transcription increases at the G1/S-phase boundary, as quiescent cells reenter the proliferative cell cycle. The growth-responsive element has been mapped to an E box at +65 in the hamster cad promoter. Using an in vivo UV cross-linking and immunoprecipitation assay, we show that Myc, Max, and upstream stimulatory factor (USF) bind to the chromosomal cad promoter. To determine whether binding of Myc-Max or USF is critical for cad growth regulation, we analyzed promoter constructs which contain mutations in the nucleotides flanking the E box. We demonstrate that altering nucleotides which flank the cad E box to sequences which decrease Myc-Max binding in vitro correlates with a loss of cad G1/S-phase transcriptional activation. This result supports the conclusion that binding of Myc-Max, but not USF, is essential for cad regulation. Our investigations demonstrate that the endogenous cad E box can be bound by more than one transcription factor, but growth-induced cad expression is achieved only by Myc.

Modifier of rudimentary p1, mod(r)p1, a trans-acting regulatory mutation of rudimentary

Modifier of rudimentaryp1 (mod(r)p1) is a hybrid dysgenesis-induced mutation isolated as a suppressor of rhd1, a hypomorphic rudimentary(r) allele in Drosophila melanogaster.mod(r)p1 has opposite effects on two of the rudimentary mutant phenotypes. It suppresses the wing truncation associated with hypomorphic r alleles, which was the phenotype used to isolate it. On the other hand, it does not suppress the sterility of r females and in fact decreases the fertility of wild-type females. This infertility is associated with a drastic decrease in r expression in mod(r)p1 females. P elementagging was used to clone the mutant allele, mod(r)p1. Subsequently, 28 kb of genomic DNA encompassing the wild-type mod(r) gene in the chromosomal region 1B was cloned. mod(r) encodes a 1.3 kb transcript which is not detected in the mod(r)p1 mutant. The sequences of mod(r) cDNA clones reveal that the gene encodes a protein of 200 amino acids in length. When compared to sequences in GenBank, the amino acid sequence did not reveal any long sequences similarities. However, the structure of the protein reveals similarities to known transcription factors. The N-terminal half of the protein is very acidic, whereas the C-terminal half is basic. The basic domain suggests a possible DNA-binding domain, while the acidic domain suggests a transcriptional activation domain. Consistent with this possibility is the fact that mod(r) acts through the 5' control region of the rudimentary gene to control its expression.