Mature lymphoid cells express the transcription repressor Bach2, which imposes regulation on humoral and cellular immunity. Here we found critical roles for Bach2 in the development of cells of the B lineage, commencing from the common lymphoid progenitor (CLP) stage, with Bach1 as an auxiliary. Overexpression of Bach2 in pre-pro-B cells deficient in the transcription factor EBF1 and single-cell analysis of CLPs revealed that Bach2 and Bach1 repressed the expression of genes important for myeloid cells ('myeloid genes'). Bach2 and Bach1 bound to presumptive regulatory regions of the myeloid genes. Bach2(hi) CLPs showed resistance to myeloid differentiation even when cultured under myeloid conditions. Our results suggest that Bach2 functions with Bach1 and EBF1 to promote B cell development by repressing myeloid genes in CLPs.

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The connection between gene regulation and metabolism is an old issue that warrants revisiting in order to understand both normal as well as pathogenic processes in higher eukaryotes. Metabolites affect the gene expression by either binding to transcription factors or serving as donors for post-translational modification, such as that involving acetylation and methylation. The focus of this review is heme, a prosthetic group of proteins that includes hemoglobin and cytochromes. Heme has been shown to bind to several transcription factors, including Bach1 and Bach2, in higher eukaryotes. Heme inhibits the transcriptional repressor activity of Bach1, resulting in the derepression of its target genes, such as globin in erythroid cells and heme oxygenase-1 in diverse cell types. Since Bach2 is important for class switch recombination and somatic hypermutation of immunoglobulin genes as well as regulatory and effector T cell differentiation and the macrophage function, the heme-Bach2 axis may regulate the immune response as a signaling cascade. We discuss future issues regarding the topic of the iron/heme-gene regulation network based on current understanding of the heme-Bach axis, including the concept of "iron immunology" as the synthesis of the iron metabolism and the immune response.

Restriction-modification systems consist of genes that encode a restriction enzyme and a cognate methyltransferase. Thus far, it was believed that restriction enzymes are sequencespecific endonucleases that introduce double-strand breaks at specific sites by catalysing the cleavages of phosphodiester bonds. Here we report that based on the crystal structure and enzymatic activity, one of the restriction enzymes, R.PabI, is not an endonuclease but a sequence-specific adenine DNA glycosylase. The structure of the R.PabI-DNA complex shows that R.PabI unwinds DNA at a 5 0 -GTAC-3 0 site and flips the guanine and adenine bases out of the DNA helix to recognize the sequence. R.PabI catalyses the hydrolysis of the N-glycosidic bond between the adenine base and the sugar in the DNA and produces two opposing apurinic/apyrimidinic (AP) sites. The opposing AP sites are cleaved by heat-promoted b elimination and/or by endogenous AP endonucleases of host cells to introduce a double-strand break.

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2), and Irf (3) families have been well studied in the context of the immune system in part due to their clear and established association with the upstream signaling cascades. In contrast, although the phosphatidylinositol 3-kinase (PI3K) pathway is critically important for the regulation of immune cells, its downstream transcription factors are still unclear except for the Foxo family (4). In this study, we investigated the possibility that the transcription factor Bach2 might be regulated in B cells by the PI3K pathway. Salient features of Bach2 relevant to this study are as follows.Bach2 regulates the immune cells at multiple points. During the development of B cells from the progenitor cells, Bach2, together with its related factor Bach1, represses the expression of myeloid genes. The repression of the myeloid program is critical for the progenitor cells to be committed to the B cell fate (5). At the stage of pre-B cells, the successful completion of antibody heavy chain gene rearrangement is monitored by the lack or presence of pre-B cell receptor (pre-BCR 2 checkpoint). Bach2 plays a critical role in negative selection (i.e. elimination of cells unsuccessful in the rearrangement) at the pre-BCR checkpoint (6). In mature B cells, Bach2 is required for the class switch recombination (CSR) and somatic hypermutation that diversify the effector function and antigen affinity, respectively, of antibody molecules in response to antigen and other stimulation (7). Bach2 promotes CSR by delaying the expression of Blimp-1, the master regulator of plasma cell differentiation, and thereby securing a time window for CSR before the terminal differentiation to plasma cells (8 -10). A reduction in the Bach2 expression in memory B cells is involved in their rapid plasma cell differentiation upon antigen re-exposure (11). An integral view of the Bach2 functions in B cells has been proposed as a gene regulatory network (GRN) consisting of Bach2 and other * This work was supported by Grants-in-aid 15H02506, 25670156, 24390066, 23116003, 21249014, 17054028, and 25291042

The transcription repressor BACH1 performs mutually independent dual roles in transcription regulation and chromosome alignment during mitosis by supporting polar ejection force of mitotic spindle. We now found that the mitotic spindles became oblique relative to the adhesion surface following endogenous BACH1 depletion in HeLa cells. This spindle orientation rearrangement was rescued by re-expression of BACH1 depending on its interactions with HMMR and CRM1, both of which are required for the positioning of mitotic spindle, but independently of its DNA-binding activity. A mass spectrometry analysis of BACH1 complexes in interphase and M phase revealed that BACH1 lost during mitosis interactions with proteins involved in chromatin and gene expression but retained interactions with HMMR and its known partners including CHICA. By analyzing BACH1 modification using stable isotope labeling with amino acids in cell culture, mitosis-specific phosphorylations of BACH1 were observed, and mutations of these residues abolished the activity of BACH1 to restore mitotic spindle orientation in knockdown cells and to interact with HMMR. Detailed histological analysis of -deficient mice revealed lengthening of the epithelial fold structures of the intestine. These observations suggest that BACH1 performs stabilization of mitotic spindle orientation together with HMMR and CRM1 in mitosis, and that the cell cycle-specific phosphorylation switches the transcriptional and mitotic functions of BACH1.