Different functions of HetR, a master regulator of heterocyst differentiation in Anabaena sp. PCC 7120, can be separated by mutation

The HetR protein has long been recognized as a key player in the regulation of heterocyst development. HetR is known to possess autoproteolytic and DNA-binding activities. During a search for mutants of Anabaena sp. PCC 7120 that can overcome heterocyst suppression caused by overexpression of the patS gene, which encodes a negative regulator of differentiation, a bypass mutant strain, S2-45, was isolated that produced a defective pattern (Pat phenotype) of irregularly spaced single and multiple contiguous heterocysts (Mch phenotype) in combined nitrogen-free medium. Analysis of the S2-45 mutant revealed a R223W mutation in HetR, and reconstruction in the wild-type background showed that this mutation was responsible for the Mch phenotype and resistance not only to overexpressed patS, but also to overexpressed hetN, another negative regulator of differentiation. Ectopic overexpression of the hetRR223W allele in the hetRR223W background resulted in a conditionally lethal (complete differentiation) phenotype. Analysis of the heterocyst pattern in the hetRR223W mutant revealed that heterocysts differentiate essentially randomly along filaments, indicating that this mutation results in an active protein that is insensitive to the major signals governing heterocyst pattern formation. These data provide genetic evidence that, apart from being an essential activator of differentiation, HetR plays a central role in the signaling pathway that controls the heterocyst pattern.     

TCP transcription factor, BRANCH ANGLE DEFECTIVE 1 (BAD1), is required for normal tassel branch angle formation in maize

In grass inflorescences, a structure called the "pulvinus" is found between the inflorescence main stem and lateral branches. The size of the pulvinus affects the angle of the lateral branches that emerge from the main axis and therefore has a large impact on inflorescence architecture. Through EMS mutagenesis we have identified three complementation groups of recessive mutants in maize having defects in pulvinus formation. All mutants showed extremely acute tassel branch angles accompanied by a significant reduction in the size of the pulvinus compared with normal plants. Two of the complementation groups correspond to mutations in the previously identified genes, RAMOSA2 (RA2) and LIGULELESS1 (LG1). Mutants corresponding to a third group were cloned using mapped-based approaches and found to encode a new member of the plant-specific TCP (TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL NUCLEAR ANTIGEN FACTOR) family of DNA-binding proteins, BRANCH ANGLE DEFECTIVE 1 (BAD1). BAD1 is expressed in the developing pulvinus as well as in other developing tissues, including the tassels and juvenile leaves. Both molecular and genetics studies show that RA2 is upstream of BAD1, whereas LG1 may function in a separate pathway. Our findings demonstrate that BAD1 is a TCP class II gene that functions to promote cell proliferation in a lateral organ, the pulvinus, and influences inflorescence architecture by impacting the angle of lateral branch emergence.     

Identification of a protein required for recovery of full antenna capacity in OCP-related photoprotective mechanism in cyanobacteria

High light can be lethal for photosynthetic organisms. Similar to plants, most cyanobacteria protect themselves from high irradiance by increasing thermal dissipation of excess absorbed energy. The photoactive soluble orange carotenoid protein (OCP) is essential for the triggering of this photoprotective mechanism. Light induces structural changes in the carotenoid and the protein, leading to the formation of a red active form. Through targeted gene interruption we have now identified a protein that mediates the recovery of the full antenna capacity when irradiance decreases. In Synechocystis PCC 6803, this protein, which we called the fluorescence recovery protein (FRP), is encoded by the slr1964 gene. Homologues of this gene are present in all of the OCP-containing strains. The FRP is a 14-kDa protein, strongly attached to the membrane, which interacts with the active red form of the OCP. In vitro this interaction greatly accelerates the conversion of the red OCP form to the orange form. We propose that in vivo, FRP plays a key role in removing the red OCP from the phycobilisome and in the conversion of the free red OCP to the orange inactive form. The discovery of FRP and its characterization are essential elements in the understanding of the OCP-related photoprotective mechanism in cyanobacteria.     

Eukaryotic initiation factor 2alpha phosphorylation is required for B-cell maturation and function in mice

Background

The control of translation initiation is a crucial component in the regulation of gene expression. The eukaryotic initiation factor 2α (eIF2α) mediates binding of the initiator transfer-messenger-RNA to the AUG initiation codon, and thus controls a rate-limiting step in translation initiation. Phosphorylation of eIF2α at serine 51 is linked to cellular stress response and attenuates translation initiation. The biochemistry of translation inhibition mediated by eIF2α phosphorylation is well characterized, yet the physiological importance in hematopoiesis remains only partially known.

Design and Methods

Using hematopoietic stem cells carrying a non-phosphorylatable mutant form of eIF2α (eIF2αAA), we examined the efficiency of reconstitution in wild-type and B-cell-deficient microMT C57BL/6 recipients in two independent models.

Results

We provide evidence that phosphorylation-deficient eIF2α mutant hematopoietic stem cells may repopulate lethally irradiated mice but have a defect in the development and maintenance of newly formed B cells in the bone marrow and of naïve follicular B cells in the periphery. The mature B-cell compartment is markedly reduced in bone marrow, spleen and peripheral blood, and B-cell receptor-mediated proliferation in vitro and serum immunoglobulin secretion in vivo are impaired.

Conclusions

The data suggest that regulation of translation through eIF2α phosphorylation is dispensable in hematopoietic reconstitution but essential during late B-cell development.