Hihara, Y. | bioRxiv Science

bioRxiv2019

cyAbrB transcriptional regulators as safety devices to inhibit heterocyst differentiation in Anabaena

Cyanobacteria are monophyletic organisms that perform oxygenic photosynthesis. While they exhibit great diversity, they have a common set of genes. However, the essentiality of them for viability has hampered the elucidation of their functions. One example of the genes is cyabrB1 encoding a transcriptional regulator. In the present study, we investigated the function of cyabrB1 in heterocyst-forming cyanobacterium Anabaena sp. PCC 7120 through CRISPR interference, a method we recently utilized for the photosynthetic production of a useful chemical in the strain. Conditional knockdown of cyabrB1 in the presence of nitrate resulted in formation of heterocysts. Two genes, hetP and hepA, which are required for heterocyst formation, were up-regulated by cyabrB1 knockdown in the presence of combined nitrogen sources. The genes are known to be induced by HetR, a master regulator of heterocyst formation. hetR was not induced by cyabrB1 knockdown. hetP and hepA were repressed by direct binding of cyAbrB1 to their promoter regions in a HetR-independent manner. In addition, the over-expression of cyabrB1 abolished heterocyst formation upon nitrogen depletion. Also, knockout of cyabrB2, a paralogue gene of cyabrB1, in addition to cyabrB1 knockdown, enhanced heterocyst formation in the presence of nitrate, suggesting functional redundancy of cyAbrB proteins. We propose that a balance between amounts of HetR and cyAbrB1 is a key factor influencing heterocyst differentiation during nitrogen step-down. cyAbrB proteins are essential safety devices inhibiting heterocyst differentiation. ImportanceSpore formation in Bacillus subtilis and Streptomyces represents non-terminal differentiation and has been extensively studied as models of prokaryotic cell differentiation. In the two organisms, many cells differentiate simultaneously, and the differentiation is governed by a network in which one regulator stands at the top. Differentiation of heterocysts in Anabaena sp. PCC 7120 has also been extensively studied. The differentiation is unique because it is terminal and only 5-10% vegetative cells differentiate into heterocysts. In the present study, we identified cyAbrB1 as a repressor of two genes that are essential for heterocyst formation, hetP and hepA, independent of HetR, which is a master activator for heterocyst differentiation. The finding is reasonable for unique cell differentiation of Anabaena because cyAbrB1 could suppress heterocyst differentiation tightly in vegetative cells, while only cells in which HetR is over-expressed could differentiate into heterocysts.

microbiology↗

bioRxiv2018

Global analysis of the RpaB regulon based on the positional distribution of HLR1 sequences and comparative differential RNA-Seq data

The transcription factor RpaB regulates the expression of genes encoding photosynthesis-associated proteins during light acclimation. The binding site of RpaB is the HLR1 motif, a pair of imperfect octameric direct repeats, separated by two random nucleotides. Here, we used high-resolution mapping data of transcriptional start sites (TSSs) in the model Synechocystis sp. PCC 6803 in conjunction with the positional distribution of HLR1 sites for the global prediction of the RpaB regulon. The results demonstrate that RpaB regulates the expression of more than 150 promoters, driving the transcription of protein-coding and non-coding genes and antisense transcripts under low light and upon the shift to high light when DNA binding activity is lost. Transcriptional activation by RpaB is achieved when the HLR1 motif is located 66 to 45 nt upstream, repression occurs when it is close to or overlapping the TSS. Selected examples were validated by multiple experimental approaches, including chromatin affinity purification, reporter gene, northern hybridization and electrophoretic mobility shift assays. We found that RpaB controls ssr2016/pgr5, which is involved in cyclic electron flow and state transitions; six out of nine ferredoxins; three of four FtsH proteases; gcvP/slr0293, encoding a crucial photorespiratory protein; and nirA and isiA for which we suggest cross-regulation with the transcription factors NtcA or FurA, respectively. In addition to photosynthetic gene functions, RpaB contributes to the control of genes affiliated with nitrogen assimilation, cofactor biosyntheses, the CRISPR system and the circadian clock, making it one of the most versatile regulators in cyanobacteria.\n\nSignificance StatementRpaB is a transcription factor in cyanobacteria and in the chloroplasts of several lineages of eukaryotic algae. Like other important transcription factors, the gene encoding RpaB cannot be deleted, making the study of deletion mutants impossible. Based on a bioinformatic approach, we increased the number of known genes controlled by RpaB by a factor of 5. Depending on the distance to the TSS, RpaB mediates transcriptional activation or repression. The high number and functional diversity among its target genes and co-regulation with other transcriptional regulators characterize RpaB as a regulatory hub.

microbiology↗