Evidence that nuclear genes code for several chloroplast ribosomal proteins

Electrophoresis of the proteins of the 50S chloroplast ribosome subunit reveals that there are at least two detectable differencesbetween Nicotiana tabacum and N. glauca. Nuclear genes contain the information for these two polypeptides since they are transmitted to F₁ interspecific hybrids independently of the maternal parent.     

Chromosomal clustering of nuclear genes encoding mitochondrial and chloroplast proteins in Arabidopsis

We present a statistical analysis of chromosomal clustering among nuclear genes encoding mitochondrial or chloroplast proteins in Arabidopsis. For both organelles, the clustering was significantly increased above the expectation, but the clustering effect was weak, and most clusters were small and dispersed. Clustered genes showed coexpression but not more than expected, and no substantial synteny was detected in other eukaryotic genomes. We propose that the unexpected clustering results from continuous selection favoring chromosomal proximity of genes acting in the same organelle.     

植物细胞核与叶绿体基因的双向调控作用

在长期进化过程中,叶绿体与细胞核已形成高度适应性。在分子水平上,两者的基因表达存在着复杂的双向调控作用。细胞核的众多基因对叶绿体基因的转录、转录后加工以及翻译等过程发挥着普遍的调节作用,而叶绿体通过四吡咯信号以及自身氧化还原状态的改变调控着细胞核基因的表达。双向调节方式的存在从分子水平上说明了细胞核与叶绿体在功能上的相互适应。然而,细胞核与叶绿体基因表达双向调控的详细机制还有待研究。     

Two nuclear mutations disrupt distinct pathways for targeting proteins to the chloroplast thylakoid.

Results of in vitro experiments have suggested the existence of at least three pathways by which nuclear-encoded proteins are targeted to the chloroplast thylakoid membrane. However, few components of the targeting machinery have been identified and the relationship between the three pathways is not clear. To investigate mechanisms underlying thylakoid protein targeting, we identified nuclear mutations in maize that cause targeting defects. We found two mutations, tha1 and hcf106, that disrupt the localization of different sets of proteins to the thylakoid lumen. The tha1 mutation interferes with the targeting of one chloroplast-encoded protein, cytochrome f, and three nuclear-encoded proteins, plastocyanin, the psaF gene product and the 33 kDa subunit of the oxygen-evolving complex. The hcf106 mutation interferes with the targeting of the 16 and 23 kDa subunits of the oxygen-evolving complex. The tha1 and hcf106 phenotypes provide the first in vivo evidence supporting the existence of two distinct thylakoid-targeting pathways. Their phenotypes also provide evidence that one chloroplast-encoded protein, cytochrome f, engages the 'tha1' pathway, indicating that nuclear- and chloroplast-encoded proteins can be targeted via common machinery.     

Hepatoma-associated non-histone proteins are phosphoproteins preferentially localized in nuclear matrix

1. High molecular weight non-histone proteins (NHP) were isolated from Morris hepatoma 7777 by Sephadex G-100, S-200 chromatography. 2. Specific polyclonal antibodies were raised against these NHP in rabbits. These antibodies recognized specific NHP components present in Morris hepatoma 7777 and 8994, but not in normal rat liver. Hepatoma-associated antigens are phosphoproteins. 3. Immunologically specific NHP of Morris hepatoma are intensively concentrated in nuclear matrix fraction.     

Dinoflagellate expressed sequence tag data indicate massive transfer of chloroplast genes to the nuclear genome

The peridinin-pigmented plastids of dinoflagellates are very poorly understood, in part because of the paucity of molecular data available from these endosymbiotic organelles. To identify additional gene sequences that would carry information about the biology of the peridinin-type dinoflagellate plastid and its evolutionary history, an analysis was undertaken of arbitrarily selected sequences from cDNA libraries constructed from Lingulodinium polyedrum (1012 non-redundant sequences) and Amphidinium carterae (2143). Among the two libraries 118 unique plastid-associated sequences were identified, including 30 (most from A. carterae) that are encoded in the plastid genome of the red alga Porphyra. These sequences probably represent bona fide nuclear genes, and suggest that there has been massive transfer of genes from the plastid to the nuclear genome in dinoflagellates. These data support the hypothesis that the peridinin-type plastid has a minimal genome, and provide data that contradict the hypothesis that there is an unidentified canonical genome in the peridinin-type plastid. Sequences were also identified that were probably transferred directly from the nuclear genome of the red algal endosymbiont, as well as others that are distinctive to the Alveolata. A preliminary report of these data was presented at the Botany 2002 meeting in Madison, WI.     

Variations in chloroplast proteins and nucleotide sequences of three chloroplast genes in Triticum and Aegilops.

Two alloplasmic wheat lines having the same common wheat nucleus but the cytoplasms of Aegilops crassa and Ae. columnaris together with the corresponding normal line (control) were used in the two-dimensional gel electrophoresis of soluble and thylakoid membrane proteins of the chloroplast. Three chloroplast polypeptides: the Rubisco large subunit, the beta subunit of ATP synthase, and an unidentified 31 kDa protein, differed in the common wheat and two Aegilops cytoplasms. Three chloroplast genes, atpB, atpE and trnM, that respectively encode the beta and epsilon subunits of ATP synthase and tRNA(met), were sequenced. The atpB gene differed by two synonymous base substitutions, whereas the other two genes were identical in the two Aegilops cytoplasms. From the predicted amino acid sequences, the beta subunits of the ATP synthase in the Aegilops cytoplasms were assumed to have three amino acid substitutions: Ala by Val, Asp- by Ala, and Gln by Lys+, in contrast to the cytoplasm of common wheat. This accounts for the difference in pI values found for the common wheat and Aegilops cytoplasms. The two base substitutions for the atpE genes of common wheat and the Aegilops cytoplasms were synonymous. The differences detected in the genes encoding the two subunits of ATP synthase do not appear to be ascribable to the differences in phenotypic effects for the common wheat and Aegilops cytoplasms. The base substitution rate of the atpB-atpE-trnM gene cluster was similar to that of the rbcL gene. From the rate for the atpB gene alone, evolutionary divergence of the wheat-Aegilops complex is assumed to have begun ca. 3.0 x 10(6) years ago, as compared to ca. 8.0 x 10(6) years ago for the divergence of the wheat-Aegilops complex and barley.     

Transport of proteins into chloroplasts. Binding of nuclear-coded chloroplast proteins to the chloroplast envelope

A system has been constructed in vitro for the binding of cytoplasmically synthesized chloroplast proteins to the chloroplast envelope which precedes the uptake into the organelle in vivo. Isolated chloroplast envelopes from young pea or spinach are capable of binding the majority of proteins obtained by translation of poly(A)-containing RNA from greening plants in vitro. Among the bound proteins the precursors to the light-harvesting chlorophyll a/b apoprotein and the small subunit of ribulose-1,5-bisphosphate carboxylase are prominent. Binding is an intrinsic property of the envelope membrane and does not require energy in the form of ATP. Bound proteins remain on the surface of the envelope vesicles and can be digested by protease. Binding is complete within minutes, shows a high affinity of the reactants, and is non-ionic in nature. Protein binding is specific for translation products of poly(A)-containing RNA from greening plants. Precursors to chloroplast protein are bound preferentially as compared to the mature proteins. The specificity is further demonstrated by the low binding of proteins obtained by run-off translation of polysomes. Binding of radioactive labeled proteins is subject to competition by excess unlabeled homologous proteins. Once bound, the proteins are withdrawn from competition indicating a high binding stability. All the properties found for binding of proteins to isolated envelopes are consistent with the concept of the so-called envelope carrier hypothesis.     

Sycamore amyloplasts can import and process precursors of nuclear encoded chloroplast proteins

Amyloplasts isolated from white-wild suspension-cultured cells of sycamore (Acer pseudoplatanus L.) are found to import and process the precursor of the small subunit (pS) of ribulose-1,5-bisphosphate carboxylase/oxygenase of spinach, but they lack the ability to form its holoenzyme due to the absence of both the large subunit and its binding-protein. They also import the precursor of the 33-kDa extrinsic protein (p33-kDa) of the O2-evolving complex of Photosystem II from spinach, but process is only to an intermediate form (i33-kDa). Chloroplasts from green-mutant cells of sycamore process p33-kDa to its mature form in this heterologous system. These results suggest that the thylakoid-associated protease responsible for the second processing step of p33-kDa is missing in amyloplasts, possibly due to the absence of thylakoid-membranes. In contrast, the apparent import of the precursor of the light-harvesting chlorophyll a/b-binding apoprotein (pLHCP) from spinach was not detected. Sycamore amyloplasts may lack the ability to import this particular thylakoid-protein, or rapidly degrade the imported molecules in the absence of thylakoid-membranes for their proper insertion.