Abscisic acid (ABA) is a plant hormone which plays an important role in seed development and dormancy and in plant response to environmental stresses. An ABA‐deficient mutant of Nicotiana plumbaginifolia, aba2, was isolated by transposon tagging using the maize Activator transposon. The aba2 mutant exhibits precocious seed germination and a severe wilty phenotype. The mutant is impaired in the first step of the ABA biosynthesis pathway, the zeaxanthin epoxidation reaction. ABA2 cDNA is able to complement N.plumbaginifolia aba2 and Arabidopsis thaliana aba mutations indicating that these mutants are homologous. ABA2 cDNA encodes a chloroplast‐imported protein of 72.5 kDa, sharing similarities with different mono‐oxigenases and oxidases of bacterial origin and having an ADP‐binding fold and an FAD‐binding domain. ABA2 protein, produced in Escherichia coli, exhibits in vitro zeaxanthin epoxidase activity. This is the first report of the isolation of a gene of the ABA biosynthetic pathway. The molecular identification of ABA2 opens the possibility to study the regulation of ABA biosynthesis and its cellular location.

SummaryThe Chlamydomonas reinhardtii nar-2, nar-3, and nar.4 genes, which are within a nitrate-regulated gene cluster containing the nitrate reductase structural gene nit-1, have been related to nitrate transport. Mutant strains defective in nitrate transport and having an active nitrate reductase have been genetically constructed. Their nitrate nonutilizing phenotype has been directly complemented by transformation using the pC0-5 plasmid which carries the nar-2, nar-3, and nar-4 clustered genes. Integration of pC0-5 DNA in the genome of nitrate transport mutants resulted in the expression of these nar transcripts and the recovery of a high affinity nitrate transport activity. Complementation of the nitrate non-utilizing phenotype of the constructed strains was also achieved by co-transformation with plasmids containing nar-2 and nar-3 genes or nar-2 and nar-4, but not with single plasmids containing each individual gene. In addition, DNA sequences of a practically complete cDNA of nar-3 and a partial one of nar-4 have been generated and the deduced amino acid sequences showed a very significant identity with that of the nitrate transporter gene (crnA) from Aspergillus nidulans. These data strongly support the hypothesis that the nitrate transport system in C. reinhardtii contains at least two protein components encoded by the nar-2 and nar-3 genes. The nar-4 gene would produce a protein with a high identity to that of nar-3.

SummaryExpression of the gene Nrt2Np, which encodes a putative high-affinity nitrate transporter of Nicotiana plumbaginifolia was studied under variable physiological conditions. Nrt2Np is rapidly induced by very low nitrate concentrations and repressed by reduced nitrogen metabolites. Furthermore, Nrt2Np is expressed in coordination with other genes involved in nitrate assimilation (Nia, Nii ). A deficiency in nitrate reductase activity, which is accompanied by high internal nitrate concentration and low levels of nitrogen metabolites, e.g. glutamine, leads to an overexpression of Nrt2Np, showing that high nitrate concentration per se does not repress Nrt2Np expression. By investigating plants with altered nitrate uptake properties, we showed a correlation between Nrt2 mRNA accumulation and 15N nitrate influx rates, providing the first evidence that the expression of Nrt2 correlates with the rate of nitrate uptake. In situ hybridization revealed a tissue-specific expression pattern. Nrt2Np mRNA accumulation is localized throughout all layers of the root tip, being highest in epidermal and endodermal cells. However, in mature root tissue, Nrt2 expression was detected mainly in the lateral root primordia and in the epidermis.

Streptococcus faecalis S-48 produces a broad spectrum antibiotic, active against Gram-positive and Gram-negative bacteria. This substance is produced in solid and liquid media and also in a defined basal medium. It is sensitive to protease, pronase, or trypsin, heating at 70 degrees C, and alkaline pH, but resistant to treatment with lipase, lysozyme, alkaline phosphatase, DNAase, RNAase, acidic or neutral pHs, and also lower temperatures (60 degrees C). Several organic solvents cause precipitation, but not inactivation. This antibiotic has been partially purified by gel filtration and further ion-exchange chromatography. Its molecular weight has been estimated close to 2000. The biological activity of this antagonistic substance against the selected indicator strains, Streptococcus faecalis S-47 and Escherichia coli U-9, is bactericidal. The characterization of this substance, initially classified as a bacteriocin, indicates that it is an antibiotic of peptidic nature. The significance of antibiotic occurrence in group D of the genus Streptococcus is also discussed.