NAB1 Is an RNA Binding Protein Involved in the Light-Regulated Differential Expression of the Light-Harvesting Antenna of Chlamydomonas reinhardtii

Abstract

Photosynthetic organisms respond to changes in ambient light by modulating the size and composition of their lightharvesting complexes, which in the case of the green alga Chlamydomonas reinhardtii consists of >15 members of a large extended family of chlorophyll binding subunits. How their expression is coordinated is unclear. Here, we describe the analysis of an insertion mutant, state transitions mutant3 (stm3), which we show has increased levels of LHCBM subunits associated with the light-harvesting antenna of photosystem II. The mutated nuclear gene in stm3 encodes the RNA binding protein NAB1 (for putative nucleic acid binding protein). In vitro and in vivo RNA binding and protein expression studies have confirmed that NAB1 differentially binds to LHCBM mRNA in a subpolysomal high molecular weight RNA-protein complex. Binding of NAB1 stabilizes LHCBM mRNA at the preinitiation level via sequestration and thereby represses translation. The specificity and affinity of binding are determined by an RNA sequence motif similar to that used by the Xenopus laevis translation repressor FRGY2, which is conserved to varying degrees in the LHCBM gene family. We conclude from our results that NAB1 plays an important role in controlling the expression of the light-harvesting antenna of photosystem II at the posttranscriptional level. The similarity of NAB1 and FRGY2 of Xenopus implies the existence of similar RNA-masking systems in animals and plants.

Changes in Transcript Abundance in Chlamydomonas reinhardtii following Nitrogen Deprivation Predict Diversion of Metabolism

“…Transcript abundance for the gene encoding NIT2, a transcription factor regulating nitrate metabolism, was increased 6-fold, while that for NAB1, a transcription factor regulating light-harvesting proteins, was decreased 16-fold, consistent with previously described physiological changes in response to N deprivation (Mussgnug et al, 2005;Camargo et al, 2007). The transcript level for the GSP1 mt + gamete-specific transcription factor was decreased 3-fold at the 48-h sample point.…”

Cysteine modification of a specific repressor protein controls the translational status of nucleus-encoded LHCII mRNAs in Chlamydomonas

“…1B). For these experiments, a probe containing the CSDCS (cold shock domain consensus motif) motif of LHCBM6 was chosen, which was previously shown to bind NAB1 specifically (4). Because of the reducing conditions used for NAB1 purification, recombinant NAB1 proteins were maintained in a reduced state.…”

“…Proteins containing a CSD motif are referred to as Y-box proteins and eukaryotic members of this large family generally contain a second auxiliary RNA-binding domain, which modulates the RNA affinity of the protein but can be dispensable for selective RNA recognition (7). In the case of NAB1, the CSD motif is combined with a C-terminal RRM (RNA recognition motif) domain, which was demonstrated not to be essential for selective RNA recognition (4). It was shown that NAB1 binds to the mRNA of LHCBM (major light-harvesting complex of photosystem II) genes, thereby preventing translation via sequestration of the message in translationally silent messenger ribonucleoprotein complexes (mRNPs).…”

“…All levels of LHCII gene expression are targeted by regulation mechanisms (2-5) which rely on a complex retrograde and anterograde communication between plastid, nucleus, and cytosol (6). The cytosolic translation repressor NAB1, which was identified in a Chlamydomonas reinhardtii light acclimation mutant (4), is the center of interest within this work. NAB1 harbors 2 RNA-binding motifs and 1 of these motifs, located at the N terminus, belongs to the highly conserved family of CSD (cold shock domain) domains.…”

“…It was shown that NAB1 binds to the mRNA of LHCBM (major light-harvesting complex of photosystem II) genes, thereby preventing translation via sequestration of the message in translationally silent messenger ribonucleoprotein complexes (mRNPs). The LHCII complex of C. reinhardtii is constituted by 10 individual highly homologous LHCBM isoforms (8,9), and NAB1 displays selectivity toward distinct isoforms with LHCBM6 mRNA being 1 of its main targets (4). It has been shown for numerous proteins that reversible modification of cysteine residues can act as an effective activity switch (10).…”