Please input one-sentence summary here. A plastid protein NUS1 is essential for build-up of the genetic system for early chloroplast development under cold stress conditions
Accumulation of NUS1 specifically occurred in the pre-emerged immature leaves, and is enhanced by low-temperature treatment. The C-terminus of NUS1 shows structural similarity to the bacterial antitermination factor NusB, which is known to play roles in the regulation of ribosomal RNA transcription. The RNA-immunoprecipitation and gel mobility shift assays indicated that NUS1 binds to several regions of chloroplast RNA including the upstream leader region of the 16S rRNA precursor. In the leaves of the NUS1-deficient mutant, accumulation of chloroplast rRNA during early leaf development was impaired and chloroplast translation/transcription capacity was severely suppressed under low temperature. Our results suggest that NUS1 is involved in the regulation of chloroplast RNA metabolism and promotes the establishment of the plastid genetic system during early chloroplast development under cold stress conditions. NUS1 encodes a previously uncharacterized chloroplast protein which has a bacterial NusB-like RNA-binding domain, and that NUS1is expressed during a strictly limited period in the early P4 stage in a temperature-dependent manner.NUS1transcript and protein accumulated abundantly in leaves at the early P4 stage and probably degraded rapidly during the late P4 stage NUS1 bound to several regions of chloroplast precursor RNAs involving the 5¢ leader region of premature 16S rRNA.In E. coli, NusB protein also interacts with the equivalent rRNA leader region (Condonet al., 1995). In the synthesis of rRNAs inE. coli, RNA polymerase undergoes several rrn operon-specific modifications that double the transcription elongation rate and allow read-through of Rhodependent terminators (Condon et al., 1995). NusB is required for this modification, and interacts with the leader and spacer regions of therrnoperon with other transcription factors such as NusA, NusE and NusG and with the ribosomal protein S4. Although we could not find genes related to Rho in the genome databases of any plant species, several homologs of bacterial Nus transcription factors seem to be present in the chloroplast. For example, NusE is the ribosomal protein S10, which has been reported to be highly conserved in bacteria and chloroplasts of higher plants (Yamaguchiet al., 2000). Ribosomal protein S4 is also conserved structurally (Russell and Bogorad, 1987; Yamaguchiet al., 2000). NUS1 may interact with these proteins and regulates rRNA synthesis during early leaf development.
V1 gene functions in the second step of chloroplast differentiation. Another important characteristic of thev1mutant is that chloroplast development was not influenced by growth temperature after leaf emergence (Iba et al., 1991). This indicates that the v1phenotype is determined at a certain stage of leaf development and that the establishment of chloroplast genetic systems is strictly regulated by the program controlling early leaf development. V1 gene, renamedNUS1, by positional cloning of the v1 mutant. We demonstrated thatNUS1 encodes previously uncharacterized chloroplast protein that has a bacterial NusB-like RNA-binding domain, and that NUS1 is expressed during a strictly limited period at the early P4 stage in a temperature-dependent manner. Our
observations suggest thatNUS1is involved in the regulation of chloroplast RNA metabolism during early leaf development, and is essential for chloroplast differentiation under low temperature.
In the chlorotic v2mutant leaves, the NUS1 protein normally accumulated in the P4 stage, and degraded in P5 (Figure S5). Accumulation of NUS1 was suppressed when seedlings were grown at 30�C, as was also observed in the wild type. This indicates that NUS1 expression is not affected by the developmental status of the chloroplast, but is strictly regulated by the intrinsic program controlling leaf development. Considering that the expression of genes related to chloroplast transcription/translation occurs concomitantly with the accumulation of NUS1 at the early P4 stage (Figure 4), and that the phenotypes of the NUS1-deficient
mutant were similar to those of the mutant plants that lack chloroplast transcription and translation .
The V1 locus was previously known as a classical phenotypic marker on the short arm of chromosome 3. Fine mapping pinpointed theV1 locus to a 40 kb region , and
identified a one-base substitution that generated a stop codon in a putative gene. The rice full-length cDNA database (http://cdna01.dna.affrc.go.jp/cDNA/) and our sequence analyses revealed that this gene consists of about 1.1 kbp of cDNA including an open reading frame (ORF) of 867 bp that encodes a protein of 280 amino acids,with a molecular mass of 32.1 kDa. A BLAST search (Altschul et al., 1990) revealed that this putative protein was previously uncharacterized and possessed a region structurally similar to the bacterial transcription termination factor NusB at its C terminus (Figure 2) (Figure S1 in Supporting Information). You can also add sub-section(s) at will.
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