The potential of nitrogen-fixing (NF) bacteria to form a symbiotic relationship with leguminous plants and fix atmospheric nitrogen has been exploited in the field to meet the nitrogen requirement of the latter. This phenomenon provides an alternative to the use of the nitrogenous fertiliser whose excessive and imbalanced use over the decades has contributed to green house emission (N2O) and underground water leaching. Recently, it was observed that non-leguminous plants like rice, sugarcane, wheat and maize form an extended niche for various species of NF bacteria. These bacteria thrive within the plant, successfully colonizing roots, stems and leaves. During the association, the invading bacteria benefit the acquired host with a marked increase in plant growth, vigor and yield. With increasing population, the demand of non-leguminous plant products is growing. In this regard, the richness of NF flora within non-leguminous plants and extent of their interaction with the host definitely shows a ray of hope in developing an ecofriendly alternative to the nitrogenous fertilisers. In this review, we have discussed the association of NF bacteria with various non-leguminous plants emphasizing on their potential to promote host plant growth and yield. In addition, plant growth-promoting traits observed in these NF bacteria and their mode of interaction with the host plant have been described briefly.

Induction of heat shock proteins (HSPs) helps cells to survive severe hyperthermal stress and removes toxic unfolded proteins. At the same time, the cap‐dependent translation of global cellular mRNA is inhibited, due to the loss of function of eukaryotic initiation factor (eIF)4F complex. It has been previously reported that, following heat shock, HSP27 binds to the insoluble granules of eIF4G and impedes its association with cytoplasmic poly(A)‐binding protein (PABP) 1 and eIF4E. In the studies reported here, in addition to heat shock, we have included results of our investigation on the association between eIF4G, PABP1 and HSP27 during recovery from heat shock, when cap‐dependent mRNA translation resumes. We showed here that in the heat‐shocked cells, the PABP1–eIF4G complex dissociated, and both polypeptides translocated with the HSP27 to the nucleus. During recovery after heat shock, PABP1 and eIF4G were redistributed into the cytoplasm and colocalized with each other. In addition, PABP1 expression was upregulated and its translation efficiency was increased during the recovery period, possibly to meet additional demands on the translation machinery. HSP27 remained associated with the eIF4G–PABP1 complex during recovery from heat shock. Therefore, our results raise the possibility that the association of HSP27 with eIF4G may not be sufficient to suppress cap‐dependent translation during heat shock. In addition, we provide evidence that the terminal oligopyrimidine cis‐element of PABP1 mRNA is responsible for the preferential increase of PABP1 mRNA translation in cells undergoing recovery from heat shock.

Regulation of gene expression at the post-transcriptional level such as control of mRNA translation and stability is of fundamental importance because it allows cells to respond quickly to external signals, and change protein synthesis without new transcriptional activity. As such, control of translation and stability of mRNA play crucial roles in a variety of cellular processes, including regulating normal cellular growth, embryogenesis and neuronal plasticity. Consequently, misregulation of mRNA translation or degradation can be associated with a number of human diseases, such as cancer and diabetes. Studies have shown that the cytoplasmic poly (A)-binding protein (PABP) plays a crucial role in regulating both translation and stability of eukaryotic mRNA.

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BackgroundIn vertebrates, poly(A) binding protein (PABP) is known to exist in five different isoforms. PABPs are primarily cytosolic with the exception of the nuclear PABP (PABPN1), which is located in the nucleus. Within the nucleus, PABPN1 is believed to bind to the poly(A) tail of nascent mRNA and along with cleavage and polyadenylation specificity factor (CPSF) define the length of the newly synthesized poly(A) tail.Methodology/Principal FindingsThe cellular role of PABP1 has been extensively studied over the years; however, the function of other PABPs remains poorly defined. In order to understand the role of PABPN1 in cellular mRNA metabolism and it’s interrelation with other PABPs, we depleted PABPN1 using RNAi in HeLa and HEK293 cells. Our results show that PABPN1 depletion did not have any effect on the poly(A) tail length, nuclear export of mRNA, mRNA translation, and transcription. Rather, PABPN1 depletion resulted in a compensatory response as observed by increased level of PABP5 and nuclear accumulation of PABP4. In addition, PABP4 was associated with the poly(A) tract of pre-mRNA and CPSF in PABPN1 depleted cells. Nevertheless, PABPN1 depletion significantly affected cell survival as evidenced by an increase in apoptosis markers: phosphorylated p53 and PUMA and as judged by the expression of ER stress marker GRP78.ConclusionOur results suggest that although function of PABPN1 may be compensated by nuclear translocation of PABP4 and perhaps by increase in the cytoplasmic abundance of PABP5, these were not sufficient to prevent apoptosis of cells. Thus PABPN1 may have a novel anti apoptotic role in mammalian cells.

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The PABPN1 [nuclear poly(A)-binding protein 1] is ubiquitous, binds to the nascent mRNA transcript and controls the poly(A) tract elongation process in multicellular organisms. Expansion of GCG repeats that encode first 6 of the 10 alanine residues of a polyalanine tract at the N-terminus of wild-type PABPN1 to 12-17 alanine residues causes aggregation of the protein and cell death. Patients with the adult onset autosomal dominant OPMD (oculopharyngeal muscular dystrophy) carry the GCG expansion mutation in their PABPN1 gene. The symptoms of OPMD include drooping eye lids and difficulty swallowing. The severity of symptoms increases with the length of the expansion. We have investigated the mechanism of cell death in HeLa and HEK-293 (human embryonic kidney) cultured cells expressing the mutant PABPN1 with a polyalanine tract containing 17 alanine residues (PABPN1-A17). In cells expressing PABPN1-A17, the abundance of pro-apoptotic proteins, p53, PUMA (p53 up-regulated modulator of apoptosis) and Noxa, are up-regulated. This was associated with the redistribution of p53 to the nucleus and mitochondria. Concomitantly Bax was translocated to the mitochondria, followed by the release of cytochrome c and the cleavage of caspase 3. Furthermore, blocking p53-mediated transcription using pifithrin significantly reduced apoptosis. Our findings suggest a key role of p53-mediated apoptosis in death of cells expressing the polyalanine expansion mutant of PABPN1.

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