The SOS response in bacteria includes a global transcriptional response to DNA damage. DNA damage is sensed by the highly conserved recombination protein RecA, which facilitates inactivation of the transcriptional repressor LexA. Inactivation of LexA causes induction (derepression) of genes of the LexA regulon, many of which are involved in DNA repair and survival after DNA damage. To identify potential RecA-LexAregulated genes in Bacillus subtilis, we searched the genome for putative LexA binding sites within 300 bp upstream of the start codons of all annotated open reading frames. We found 62 genes that could be regulated by putative LexA binding sites. Using mobility shift assays, we found that LexA binds specifically to DNA in the regulatory regions of 54 of these genes, which are organized in 34 putative operons. Using DNA microarray analyses, we found that 33 of the genes with LexA binding sites exhibit RecA-dependent induction by both mitomycin C and UV radiation. Among these 33 SOS genes, there are 22 distinct LexA binding sites preceding 18 putative operons. Alignment of the distinct LexA binding sites reveals an expanded consensus sequence for the B. subtilis operator: 5-CGAACATATGTTCG-3. Although the number of genes controlled by RecA and LexA in B. subtilis is similar to that of Escherichia coli, only eight B. subtilis RecA-dependent SOS genes have homologous counterparts in E. coli.Exposure of prokaryotes to DNA-damaging agents results in the induction of a diverse set of physiological responses collectively called the SOS response (8, 55). As first characterized in Escherichia coli, the SOS response includes an enhanced capacity for recombinational repair, enhanced capacity for excision repair, enhanced mutagenesis (due to error-prone repair), and inhibition of cell division (i.e., filamentation). Induction of the SOS response is due to the coordinate derepression of a number of SOS or din (for damage-inducible) genes. The SOS response to DNA damage in Bacillus subtilis is similar to that of E. coli (26,56,58), but unlike E. coli, the B. subtilis SOS system is also induced in competent cells in the absence of any DNA-damaging treatment (25, 57, 58). As in E. coli, SOS gene expression in B. subtilis is controlled by two proteins (which are themselves products of SOS genes): the LexA protein (also called DinR) (40,54), which represses the transcription of din genes by binding to the SOS operator (31), and the RecA protein (30), which is activated by single-stranded DNA (29,42) to stimulate the proteolytic autodigestion of LexA (24,31). Thus, an SOS gene is defined by two criteria-RecA-dependent induction by DNA damage and a binding site for LexA overlapping its promoter.By contrast with E. coli, where more than 30 SOS genes have been identified (7,8), only 5 B. subtilis SOS genes have been shown to meet both SOS gene criteria thus far: recA, lexA, uvrB (formerly dinA), dinB, and dinC (also called tagC) (4,9,15,25).
Several biological processes in Trypanosoma brucei are affected by chromatin structure, including gene expression, cell cycle regulation, and life-cycle stage differentiation. In Saccharomyces cerevisiae and other organisms, chromatin structure is dependent upon posttranslational modifications of histones, which have been mapped in detail. The tails of the four core histones of T. brucei are highly diverged from those of mammals and yeasts, so sites of potential modification cannot be reliably inferred, and no cross-species antibodies are available to map the modifications. We therefore undertook an extensive survey to identify posttranslational modifications by Edman degradation and mass spectrometry. Edman analysis showed that the N-terminal alanine of H2A, H2B, and H4 could be monomethylated. We found that the histone H4 N-terminus is heavily modified, while, in contrast to other organisms, the histone H2A and H2B N-termini have relatively few modifications. Histone H3 appears to have a number of modifications at the N-terminus, but we were unable to assign many of these to a specific amino acid. Therefore, we focused our efforts on uncovering modification states of H4. We discuss the potential relevance of these modifications.
Some inroads have been made into characterizing histone variants and post translational modifications of histones in Trypanosoma brucei. Histone variant H2BV lysine 129 is homologous to Saccharomyces cerevisiae H2B lysine 123, whose ubiquitination is required for methylation of H3 lysines 4 and 79. We show that T. brucei H2BV K129 is not ubiquitinated, but trimethylation of H3 K4 and K76, homologs of H3 K4 and K79 in yeast, was enriched in nucleosomes containing H2BV. Mutation of H2BV K129 to alanine or arginine did not disrupt H3 K4 or K76 methylation. These data suggest that H3 K4 and K76 methylation in trypanosomes is regulated by a novel mechanism, possibly involving the replacement of H2B with H2BV in the nucleosome.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.