Cell-specific regulation of the CD21 gene.

The Complement Receptor Type 2 (Cr2-145,CR2, CD21) is an important receptor in the innate and acquired immune response. CD21 is produced by B cells and follicular dendritic cells, where it binds cleavage products of the C3 complement protein. CD21 facilitates internalization of immune complexes by B cells to enhance antigen presentation. CD21, in association with CD19/CD81, also serves as a coaccessory activation complex with the B-cell antigen receptor, permitting a lower antigen concentration to achieve maximal B-cell activation. CD21 traps immune complexes on the surface of follicular dendritic cells and displays them to activated B cells in germinal centers. Much work has been conducted to determine the transcriptional control mechanisms dictating CD21 expression. Appropriate transcriptional control of the CD21 gene evidently requires the CD21 promoter, as well as intronic sequences with enhancer and suppressor functions. Chromatin structure has been implicated in regulating the coordination of CD21 promoter and intronic control sequences by regulating access to them by putative transcription factors. This review assesses the past and current research into CD21 transcriptional regulation and offers insight into future experimental directions.

Cell-specific expression of the murine CD21 gene depends on accessibility of promoter and intronic elements.

The murine complement receptor type 2 gene (Cr2/CD21) is transcriptionally active in murine B and follicular dendritic cells, but not in murine T cells. We have previously shown that altering chromatin structure via histone deacetylase inhibitors results in CD21 expression in murine T cells, and that the minimal CD21 promoter provided appropriate cell-specific expression of luciferase reporter constructs only in the presence of the first third of intron 1, fragment A. We extend this work by showing that replacing the CD21 gene promoter with the SV40 promoter resulted in the loss of this cell-specific control. Further delineation of intronic regulatory elements by fragmentation also resulted in the loss of cell-specific gene expression, suggesting that multiple CD21 promoter and intronic elements interact for appropriate CD21 gene expression. To assess this model, we performed EMSAs to define protein binding sites within promoter and intronic regions and DNase I hypersensitivity assays to determine chromatin accessibility. Multiple DNA binding factors were shown to be present in B and T cell extracts; a minority demonstrated B cell specificity. However, the DNase I sensitivity of T cell CD21 regulatory elements was not comparable to that of B cells until the histone acetylation status of the gene was altered. Taken together, these data suggest that chromatin remodeling facilitates cell-specific CD21 gene expression by modulating access of transcription factors to regulatory elements in the promoter and intron.

Regulation of the thdF gene, which is involved in thiophene oxidation by Escherichia coli K-12.

The thdF gene of Escherichia coli encodes a 48 kD protein which is involved in the oxidation of derivatives of the sulphur-containing heterocycle thiophene and which appears to be induced during stationary phase. In this work the upstream regulatory region of the thdF gene was isolated by polymerase chain reaction and inserted in front of the lacZ structural gene. Examination of the resulting thdF-lacZ operon fusions showed that expression of the thdF gene increased as E. coli entered the stationary phase. However, the expression of thdF was not dependent on RpoS (KatF), the stationary phase sigma factor. The thdF gene was subject to substantial catabolite repression by glucose and its expression was also greatly decreased in the absence of oxygen. The thdF-lacZ fusions were not significantly affected by elevated temperature or medium of high osmolarity, nor by mutations in thdA, fadR, arcA, arcB, or fnr. Both multicopy, plasmid-borne fusions and single-copy fusions gave similar results in all of the above cases except that the plasmid-borne fusions still showed substantial expression in the absence of oxygen. The heterocyclic compounds thiophene carboxylic acid, furan carboxylic acid and proline increased expression of the thdF gene by 2- to 3-fold, but only during the stationary phase. Tryptophan, indole, and several indole derivatives had no effect.

Lymphoid transcription of the murine CD21 gene is positively regulated by histone acetylation.

Intronic transcriptional control sequences influence the cell- and tissue-specific expression of the CD21 gene. The interactions of such intronic control sequences, which are physically separated from the gene's promoters, suggest that factors that alter chromatin structure might be influential in this process. Accordingly, we analyzed the effect of histone acetylation on the expression of CD21 in nonexpressing T and B lymphocytes, respectively. The acetylase inhibitors sodium butyrate and trichostatin A were used to create hyperacetylated histones. The CD21 gene was specifically activated in the previously transcriptionally quiescent cells in a time- and dose-dependent fashion: the expression of a number of other genes was not influenced. These data suggest a model of cell-type-specific deacetylase activity that serves to repress gene transcription when present and active.