Characterization of a cDNA encoding a novel plant poly(A) polymerase

A chloroplast cyclophilin functions in the assembly and maintenance of photosystem II in Arabidopsis thaliana

Photosynthetic light reactions rely on the proper function of large protein complexes (including photosystems I and II) that reside in the thylakoid membrane. Although their composition, structure, and function are known, the repertoire of assembly and maintenance factors is still being determined. Here we show that an immunophilin of the cyclophilin type, CYP38, plays a critical role in the assembly and maintenance of photosystem II (PSII) supercomplexes (SCs) in Arabidopsis. Mutant plants with the CYP38 gene interrupted by T-DNA insertion showed stunted growth and were hypersensitive to high light. Leaf chlorophyll fluorescence analysis and thylakoid membrane composition indicated that cyp38 mutant plants had defects in PSII SCs. Sucrose supplementation enabled the rescue of the mutant phenotype under low-light conditions, but failed to mitigate hypersensitivity to high-light stress. Protein radiolabeling assays showed that, although individual thylakoid proteins were synthesized equally in mutant and wild type, the assembly of the PSII SC was impaired in the mutant. In addition, the D1 and D2 components of the mutant PSII had a short half-life under high-light stress. The results provide evidence that CYP38 is necessary for the assembly and stabilization of PSII.

3'-end formation of baculovirus late RNAs

Baculovirus late RNAs are transcribed by a four-subunit RNA polymerase that is virus encoded. The late viral mRNAs are capped and polyadenylated, and we have previously shown that capping is mediated by the LEF-4 subunit of baculovirus RNA polymerase. Here we report studies undertaken to determine the mechanism of 3'-end formation. A globin cleavage/polyadenylation signal, which was previously shown to direct 3'-end formation of viral RNAs in vivo, was cloned into a baculovirus transcription template. In vitro assays with purified baculovirus RNA polymerase revealed that 3' ends were formed not by a cleavage mechanism but rather by termination after transcription of a T-rich region of the globin sequence. Terminated RNAs were released from ternary complexes and were subsequently polyadenylated. Mutational analyses indicated that the T-rich sequence was essential for termination and polyadenylation, but the poly(A) signal and the GT-rich region of the globin polyadenylation/cleavage signal were not required. Termination was not dependent on ATP hydrolysis, indicating a slippage mechanism.

Nuclear and chloroplast poly(A) polymerases from plants share a novel biochemical property

Poly(A) polymerases are centrally involved in the process of mRNA 3' end formation in eukaryotes. In animals and yeast, this enzyme works as part of a large multimeric complex to add polyadenylate tracts to the 3' ends of precursor RNAs in the nucleus. Plant nuclear enzymes remain largely uncharacterized. In this report, we describe an initial analysis of plant nuclear poly(A) polymerases (nPAPs). An enzyme purified from pea nuclear extracts possesses many features that are seen with the enzymes from yeast and mammals. However, the pea enzyme possesses the ability to polyadenylate RNAs that are associated with polynucleotide phosphorylase (PNP), a chloroplast-localized enzyme involved in RNA turnover. Similar behavior is not seen with the yeast poly(A) polymerase (PAP). A fusion protein consisting of glutathione-S-transferase and the active domain of an Arabidopsis-encoded nuclear poly(A) polymerase was also able to utilize PNP, indicating that the activity of the pea enzyme was due to an interaction between the pea nPAP and PNP, and not to other factors that might copurify with the pea enzyme. These results suggest the existence, in plant nuclei, of factors related to PNP, and an interaction between such factors and poly(A) polymerases.

Formation of mRNA 3' ends in eukaryotes: mechanism, regulation, and interrelationships with other steps in mRNA synthesis

Formation of mRNA 3' ends in eukaryotes requires the interaction of transacting factors with cis-acting signal elements on the RNA precursor by two distinct mechanisms, one for the cleavage of most replication-dependent histone transcripts and the other for cleavage and polyadenylation of the majority of eukaryotic mRNAs. Most of the basic factors have now been identified, as well as some of the key protein-protein and RNA-protein interactions. This processing can be regulated by changing the levels or activity of basic factors or by using activators and repressors, many of which are components of the splicing machinery. These regulatory mechanisms act during differentiation, progression through the cell cycle, or viral infections. Recent findings suggest that the association of cleavage/polyadenylation factors with the transcriptional complex via the carboxyl-terminal domain of the RNA polymerase II (Pol II) large subunit is the means by which the cell restricts polyadenylation to Pol II transcripts. The processing of 3' ends is also important for transcription termination downstream of cleavage sites and for assembly of an export-competent mRNA. The progress of the last few years points to a remarkable coordination and cooperativity in the steps leading to the appearance of translatable mRNA in the cytoplasm.