Functional analysis of salt-inducible proline transporter of barley roots.

We cloned a cDNA encoding Hordeum vulgare Proline Transporter (HvProT) from salt-stressed barley roots by differential display. HvProT was 2,161 bp long and had an open reading frame encoding 450 amino acids. The deduced amino acid sequence of HvProT was similar to those of proline transporter proteins of rice (65.7%), Arabidopsis (57.7%) and tomato (42.0%). Northern blot analysis showed that the transcript level of HvProT was induced in roots at 30 min after 200 mM NaCl treatment and its peak was observed at 3 h. However, the transcript level was very low in leaves and did not increase by salt stress. The expression level of Delta(1)-pyrroline-5-carboxylate synthetase (P5CS), encoding a key enzyme of proline synthesis, was induced later than HvProT by salt stress. A transport assay using a yeast with mutation in proline uptake revealed that HvProT was a transporter with high affinity for L-proline (K(m) = 25 microM). HvProT was found to be a unique transporter with high affinity for L-proline. Since its transport activity was dependent on the pH gradient, HvProT was suggested to be a H(+)/amino acid symporter. In situ hybridization analysis showed that the HvProT mRNA was strongly expressed in root cap cells under salt stress. HvProT might play an important role in the transport of proline to root tip region urgently upon salt stress.

Localization of farnesyl diphosphate synthase in chloroplasts.

The subcellular localization of plant farnesyl diphosphate synthase (FPPS) was examined. Immunocytochemical staining using anti-FPPS1 antibody followed by electron microscopy showed that FPPS1 was localized to chloroplasts of rice mesophyll cells. Subcellular fractions from wheat leaves were examined by immunoblot analysis. FPPS was detected in the chloroplast fraction in wheat, and was protected from proteolysis following trypsin treatment of chloroplasts. FPPS was also detected in the chloroplast fraction of a dicot plant, tobacco.

A novel importin alpha from rice, a component involved in the process of nuclear protein transport.

In eukaryotes, nuclear proteins that are transported into nuclei have nuclear localization signals (NLSs), which are recognized by proteins called importin alpha. We isolated a rice cDNA, #61L, and the corresponding gene that encodes a protein, which shows significant homology to the importin alpha. Although the encoded protein had only 23-27% amino acid identity to the importin alphas from various organisms including plants, the fusion protein with glutathione S-transferase showed a specific binding activity to the NLS of SV40 T-antigen. These results suggest that the rice #61L protein is a novel importin alpha in plants.

Cloning of a cDNA that encodes farnesyl diphosphate synthase and the blue-light-induced expression of the corresponding gene in the leaves of rice plants.

A cDNA encoding farnesyl diphosphate synthase (FPPS), a key enzyme in isoprenoid biosynthesis, was isolated from a cDNA library constructed from mRNA that had been prepared from etiolated rice (Oriza sativa L. variety Nipponbare) seedlings after three hours of illumination by a subtraction method. The putative polypeptide deduced from the 1289 bp nucleotide sequence consisted of 353 amino acids and had a molecular mass of 40 676 Da. The predicted amino acid sequence exhibited high homology to those of FPPS from Arabidopsis (73% to type 1, 72% to type 2) and white lupin (74%). Southern blot analysis showed that the rice genome might contain only one gene for FPPS. The highest level of expression of the gene was demonstrated in leaves by RNA blot analysis. Moreover, light, in particular blue light, effectively enhanced expression of the gene.

cDNA cloning of squalene synthase genes from mono- and dicotyledonous plants, and expression of the gene in rice.

cDNA clones encoding squalene synthases were isolated from rice, maize and soybeans. A phylogenetic tree showed that the enzymes of monocots and dicots form distinct subgroups. In rice, squalene synthase mRNA was detected in tissues containing dividing cells and its level was repressed by illumination.

Integration of bacteriophage Mx8 into the Myxococcus xanthus chromosome causes a structural alteration at the C-terminal region of the IntP protein.

Mx8 is a generalized transducing phage that infects Myxococcus xanthus cells. This phage is lysogenized in M. xanthus cells by the integration of its DNA into the host chromosome through site-specific recombination. Here, we characterize the mechanism of Mx8 integration into the M. xanthus chromosome. The Mx8 attachment site, attP, the M. xanthus chromosome attachment site, attB, and two phage-host junctions, attL and attR, were cloned and sequenced. Sequence alignments of attP, attB, attL, and attR sites revealed a 29-bp segment that is absolutely conserved in all four sequences. The intP gene of Mx8 was found to encode a basic protein that has 533 amino acids and that carries two domains conserved in site-specific recombinases of the integrase family. Surprisingly, the attP site was located within the coding sequence of the intP gene. Hence, the integration of Mx8 into the M. xanthus chromosome results in the conversion of the intP gene to a new gene designated intR. As a result of this conversion, the 112-residue C-terminal sequence of the intP protein is replaced with a 13-residue sequence. A 3-base deletion within the C-terminal region had no effect on Mx8 integration into the chromosome, while a frameshift mutation with the addition of 1 base at the same site blocked integration activity. This result indicates that the C-terminal region is required for the enzymatic function of the intP product.