GroEL of the nitrogen-fixing cyanobacterium Anabaena sp. strain L-31 exhibits GroES and ATP-independent refolding activity

Proteomics analysis reveals differential acclimation of coastal and oceanic Synechococcus to climate warming and iron limitation

In many oceanic regions, anthropogenic warming will coincide with iron (Fe) limitation. Interactive effects between warming and Fe limitation on phytoplankton physiology and biochemical function are likely, as temperature and Fe availability affect many of the same essential cellular pathways. However, we lack a clear understanding of how globally significant phytoplankton such as the picocyanobacteria Synechococcus will respond to these co-occurring stressors, and what underlying molecular mechanisms will drive this response. Moreover, ecotype-specific adaptations can lead to nuanced differences in responses between strains. In this study, Synechococcus isolates YX04-1 (oceanic) and XM-24 (coastal) from the South China Sea were acclimated to Fe limitation at two temperatures, and their physiological and proteomic responses were compared. Both strains exhibited reduced growth due to warming and Fe limitation. However, coastal XM-24 maintained relatively higher growth rates in response to warming under replete Fe, while its growth was notably more compromised under Fe limitation at both temperatures compared with YX04-1. In response to concurrent heat and Fe stress, oceanic YX04-1 was better able to adjust its photosynthetic proteins and minimize the generation of reactive oxygen species while reducing proteome Fe demand. Its intricate proteomic response likely enabled oceanic YX04-1 to mitigate some of the negative impact of warming on its growth during Fe limitation. Our study highlights how ecologically-shaped adaptations in Synechococcus strains even from proximate oceanic regions can lead to differing physiological and proteomic responses to these climate stressors.

Evolution of Chaperonin Gene Duplication in Stigonematalean Cyanobacteria (Subsection V)

Chaperonins promote protein folding and are known to play a role in the maintenance of cellular stability under stress conditions. The group I bacterial chaperonin complex comprises GroEL, that forms a barrel-like oligomer, and GroES that forms the lid. In most eubacteria the GroES/GroEL chaperonin is encoded by a single-copy bicistronic operon, whereas in cyanobacteria up to three groES/groEL paralogs have been documented. Here we study the evolution and functional diversification of chaperonin paralogs in the heterocystous, multi-seriate filament forming cyanobacterium Chlorogloeopsis fritschii PCC 6912. The genome of C. fritschii encodes two groES/groEL operons (groESL1, groESL1.2) and a monocistronic groEL gene (groEL2). A phylogenetic reconstruction reveals that the groEL2 duplication is as ancient as cyanobacteria, whereas the groESL1.2 duplication occurred at the ancestor of heterocystous cyanobacteria. A comparison of the groEL paralogs transcription levels under different growth conditions shows that they have adapted distinct transcriptional regulation. Our results reveal that groEL1 and groEL1.2 are upregulated during diazotrophic conditions and the localization of their promoter activity points towards a role in heterocyst differentiation. Furthermore, protein–protein interaction assays suggest that paralogs encoded in the two operons assemble into hybrid complexes. The monocistronic encoded GroEL2 is not forming oligomers nor does it interact with the co-chaperonins. Interaction between GroES1.2 and GroEL1.2 could not be documented, suggesting that the groESL1.2 operon does not encode a functional chaperonin complex. Functional complementation experiments in Escherichia coli show that only GroES1/GroEL1 and GroES1/GroEL1.2 can substitute the native operon. In summary, the evolutionary consequences of chaperonin duplication in cyanobacteria include the retention of groESL1 as a housekeeping gene, subfunctionalization of groESL1.2 and neofunctionalization of the monocistronic groEL2 paralog.

Myxococcus xanthus DK1622 Coordinates Expressions of the Duplicate groEL and Single groES Genes for Synergistic Functions of GroELs and GroES

Chaperonin GroEL (Cpn60) requires cofactor GroES (Cpn10) for protein refolding in bacteria that possess single groEL and groES genes in a bicistronic groESL operon. Among 4,861 completely-sequenced prokaryotic genomes, 884 possess duplicate groEL genes and 770 possess groEL genes with no neighboring groES. It is unclear whether stand-alone groEL requires groES in order to function and, if required, how duplicate groEL genes and unequal groES genes balance their expressions. In Myxococcus xanthus DK1622, we determined that, while duplicate groELs were alternatively deletable, the single groES that clusters with groEL1 was essential for cell survival. Either GroEL1 or GroEL2 required interactions with GroES for in vitro and in vivo functions. Deletion of groEL1 or groEL2 resulted in decreased expressions of both groEL and groES; and ectopic complementation of groEL recovered not only the groEL but also groES expressions. The addition of an extra groES gene upstream groEL2 to form a bicistronic operon had almost no influence on groES expression and the cell survival rate, whereas over-expression of groES using a self-replicating plasmid simultaneously increased the groEL expressions. The results indicated that M. xanthus DK1622 cells coordinate expressions of the duplicate groEL and single groES genes for synergistic functions of GroELs and GroES. We proposed a potential regulation mechanism for the expression coordination.