Transposable element-induced mutations of the viviparous-1 gene in maize

The viviparous-1 (vp1) locus in maize is a developmental gene that controls diverse aspects of the maturation phase of seed development. Mutations of vp1 alter embryo sensitivity to the hormone abscisic acid and block formation of anthocyanin pigment. Molecular cloning of a Robertson Mutator-induced mutant allele, vp1-mum-1, by transposable element tagging has allowed analysis of several transposon-induced vp1 mutants. In the vp1-Mc mutation, the gene is disrupted by 4.0 kbp insertion, which results in expression of a 3′ truncated mRNA. Phenotypically, this allele is at least partially functional in causing embryo dormancy, but is ineffective in controlling anthocyanin expression. This result suggests that disruption of the C-terminal domain of the Vp1 protein specifically affects regulation of the anthocyanin pathway. A second Mutator- derived allele, vp1-mum2, exhibits an unusual form of somatic mutability in which endosperm cells revert from wild-type vp1 expression to a mutant condition. The vp1-mum2 allele contains a 1.5 kbp Insertion that has no detectable homology to known Mu elements. This element is retained In wild-type germinal revertants derived from vp1-mum2 An apparent DNA modification affecting cleavage at an internal Sstl restriction site in the element correlates with vp1-mum2 states that exhibit wild-type Vp1 expression. A model involving mitotic assortment of modified and unmodified DNA strands during development is proposed for vp1-mum2 somatic mutation.     

Inhibition of ribulose-1,5-bisphosphate carboxylase/oxygenase by ribulose-1,5-bisphosphate epimerization and degradation products

Xylulose-1,5-bisphosphate in preparations of ribulose-1,5-bisphosphate (ribulose-P₂) arises from non-enzymic epimerization and inhibits the enzyme. Another inhibitor, a diketo degradation product from ribulose-P₂, is also present. Both compounds simulate the substrate inhibition of ribulose-P₂ carboxylase/oxygenase previously reported for ribulose-P₂. Freshly prepared ribulose-P₂ had little inhibitory activity. The instability of ribulose-P₂ may be one reason for a high level of ribulose-P₂ carboxylase in chloroplasts where the molarity of active sites exceeds that of ribulose-P₂. Because the K_D of the enzyme/substrate complex is ≤1 μM, all ribulose-P₂ generated in situ may be stored as this complex to prevent decomposition.