DrpB (YedR) Is a Nonessential Cell Division Protein in Escherichia coli.

We report that the small Escherichia coli membrane protein DrpB (formerly YedR) is involved in cell division. We discovered DrpB in a screen for multicopy suppressors of a ΔftsEX mutation that prevents divisome assembly when cells are plated on low ionic strength medium, such as lysogeny broth without NaCl. Characterization of DrpB revealed that (i) translation initiates at an ATG annotated as codon 22 rather than the GTG annotated as codon 1, (ii) DrpB localizes to the septal ring when cells are grown in medium of low ionic strength but localization is greatly reduced in medium of high ionic strength, (iii) overproduction of DrpB in a ΔftsEX mutant background improves recruitment of the septal peptidoglycan synthase FtsI, implying multicopy suppression works by rescuing septal ring assembly, (iv) a ΔdrpB mutant divides quite normally, but a ΔdrpB ΔdedD double mutant has a strong division and viability defect, albeit only in medium of high ionic strength, and (v) DrpB homologs are found in E. coli and a few closely related enteric bacteria, but not outside this group. In sum, DrpB is a poorly conserved nonessential division protein that improves the efficiency of cytokinesis under suboptimal conditions. Proteins like DrpB are likely to be a widespread feature of the bacterial cell division apparatus, but they are easily overlooked because mutants lack obvious shape defects.IMPORTANCE A thorough understanding of bacterial cell division requires identifying and characterizing all of the proteins that participate in this process. Our discovery of DrpB brings us one step closer to this goal in E. coli.

Light-dependent attenuation of phycoerythrin gene expression reveals convergent evolution of green light sensing in cyanobacteria.

The colorful process of chromatic acclimation allows many cyanobacteria to change their pigmentation in response to ambient light color changes. In red light, cells produce red-absorbing phycocyanin (PC), whereas in green light, green-absorbing phycoerythrin (PE) is made. Controlling these pigment levels increases fitness by optimizing photosynthetic activity in different light color environments. The light color sensory system controlling PC expression is well understood, but PE regulation has not been resolved. In the filamentous cyanobacterium Fremyella diplosiphon UTEX 481, two systems control PE synthesis in response to light color. The first is the Rca pathway, a two-component system controlled by a phytochrome-class photoreceptor, which transcriptionally represses cpeCDESTR (cpeC) expression during growth in red light. The second is the Cgi pathway, which has not been characterized. We determined that the Cgi system also regulates PE synthesis by repressing cpeC expression in red light, but acts posttranscriptionally, requiring the region upstream of the CpeC translation start codon. cpeC RNA stability was comparable in F. diplosiphon cells grown in red and green light, and a short transcript that included the 5' region of cpeC was detected, suggesting that the Cgi system operates by transcription attenuation. The roles of four predicted stem-loop structures within the 5' region of cpeC RNA were analyzed. The putative stem-loop 31 nucleotides upstream of the translation start site was required for Cgi system function. Thus, the Cgi system appears to be a unique type of signal transduction pathway in which the attenuation of cpeC transcription is regulated by light color.

Functional characterization of a cyanobacterial OmpR/PhoB class transcription factor binding site controlling light color responses.

Complementary chromatic acclimation (CCA) allows many cyanobacteria to change the composition of their light-harvesting antennae for maximal absorption of different wavelengths of light. In the freshwater species Fremyella diplosiphon, this process is controlled by the ratio of red to green light and allows the differential regulation of two subsets of genes in the genome. This response to ambient light color is controlled in part by a two-component system that includes a phytochrome class photoreceptor and a response regulator with an OmpR/PhoB class DNA binding domain called RcaC. During growth in red light, RcaC is able to simultaneously activate expression of red light-induced genes and repress expression of green light-induced genes through binding to the L box promoter element. Here we investigate how the L box functions as both an activator and a repressor under the same physiological conditions by analyzing the effects of changing the position, orientation, and sequence of the L box. We demonstrate that changes in the local sequences surrounding the L box affect the strength of its activity and that the activating and repressing functions of the L box are orientation dependent. Also, the spacing between the L box and the transcription start site is critical for it to work as an activator, while its repressing role during light regulation requires additional upstream and downstream DNA sequence elements. The latter result suggests that the repressing function of RcaC requires it to operate in association with multiple additional DNA binding proteins, at least one of which is functioning as an activator.

Inverse transcriptional activities during complementary chromatic adaptation are controlled by the response regulator RcaC binding to red and green light-responsive promoters.

Complementary chromatic adaptation (CCA) provides cyanobacteria with the ability to shift between red and blue-green phenotypes that are optimized for absorption of different wavelengths of light. Controlled by the ratio of green to red light, this process results from differential expression of two groups of operons, many of which encode proteins involved in photosynthetic light harvesting antennae biogenesis. In the freshwater species Fremyella diplosiphon, the inverse regulation of these two classes is complex and occurs through different mechanisms. It also involves a two-component pathway that includes a phytochrome-class photoreceptor and the response regulator RcaC. Here we uncover the mechanism through which this system controls CCA by demonstrating that RcaC binds to the L Box within promoters of both classes of light-regulated operons. We provide functional evidence that complementary regulation of these operons occurs by RcaC's simultaneous activation and repression of transcription in red light. We identify rcaC and L Boxes in the genome of a marine cyanobacterium capable of CCA, suggesting widespread use of this control system. These results provide important insights into the long-standing enigma of CCA regulation and complete the first description of an entire two-component system controlled by a phytochrome-class photoreceptor.

A light regulated OmpR-class promoter element co-ordinates light-harvesting protein and chromophore biosynthetic enzyme gene expression.

Co-ordination of chromophore and apoprotein biosynthesis is required during photosynthetic light-harvesting antennae production, such as occurs during complementary chromatic adaptation (CCA). This response to ambient light colour changes is controlled by a phytochrome-class photoreceptor and involves changes in the synthesis of cyanobacterial light-harvesting antennae. During growth in red light, CCA activates cpc2 transcription, an operon that encodes the light-harvesting protein phycocyanin. In order to function, this apoprotein must have covalently attached phycocyanobilin chromophores, which are synthesized by PcyA. We show that pcyA is also transcriptionally activated by CCA during red light growth and is not regulated via feedback that senses cpc2 RNA levels. The pcyA and cpc2 promoters contain a common regulatory element, a direct repeat typical of OmpR-class transcription factor binding sites, at similar positions relative to their red light-controlled transcription start sites. Deletion of this element from the pcyA promoter eliminated CCA-regulated transcription, and insertion of the element into a non-light responsive promoter conferred CCA regulation. We conclude that this element is necessary and sufficient to confer CCA transcriptional regulation and that it co-ordinates phycocyanin and phycocyanobilin biosynthesis in red light.