The Architecture and Conservation Pattern of Whole-Cell Control Circuitry

Department of Developmental Biology, Stanford University School of Medicine, Stanford University, Stanford, CA 94305, USA.
Journal of Molecular Biology (Impact Factor: 4.33). 02/2011; 409(1):28-35. DOI: 10.1016/j.jmb.2011.02.041
Source: PubMed


The control circuitry that directs and paces Caulobacter cell cycle progression involves the entire cell operating as an integrated system. This control circuitry monitors the environment and the internal state of the cell, including the cell topology, as it orchestrates orderly activation of cell cycle subsystems and Caulobacter's asymmetric cell division. The proteins of the Caulobacter cell cycle control system and its internal organization are co-conserved across many alphaproteobacteria species, but there are great differences in the regulatory apparatus' functionality and peripheral connectivity to other cellular subsystems from species to species. This pattern is similar to that observed for the "kernels" of the regulatory networks that regulate development of metazoan body plans. The Caulobacter cell cycle control system has been exquisitely optimized as a total system for robust operation in the face of internal stochastic noise and environmental uncertainty. When sufficient details accumulate, as for Caulobacter cell cycle regulation, the system design has been found to be eminently rational and indeed consistent with good design practices for human-designed asynchronous control systems.

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    • "A recent review (McAdams & Shapiro, 2011) describes a fifth (Christen et al., 2011; Tan et al., 2010) regulator, SciP, a CtrA antagonist that inhibits the transcription of at least 58 CtrA-activated genes (Gora et al., 2010; Tan et al., 2010) in swarmer cells, limiting their expression to the predivisional stage of the cell cycle. SciP integrates with the oscillating core cell cycle regulators via CtrA and CcrM (Gora et al., 2010; Tan et al., 2010). "
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    ABSTRACT: The master regulator CtrA oscillates during the Caulobacter cell cycle due to temporally regulated proteolysis and transcription. It is proteolysed during the G1-S transition and reaccumulates in predivisional cells as a result of transcription from two sequentially activated promoters, P1 and P2. CtrA reinforces its own synthesis by directly mediating the activation of P2 concurrently with repression of P1. To explore the role of P1 in cell cycle control, we engineered a mutation into the native ctrA locus that prevents transcription from P1 but not P2. As expected, the ctrA P1 mutant exhibits striking growth, morphological and DNA replication defects. Unexpectedly, we found CtrA and its antagonist SciP, but not DnaA, GcrA or CcrM accumulation to be dramatically reduced in the ctrA P1 mutant. SciP levels closely paralleled CtrA accumulation, suggesting that CtrA acts as a rheostat to modulate SciP abundance. Furthermore, the reappearance of CtrA and CcrM in predivisional cells was delayed in the P1 mutant by 0.125 cell cycle unit in synchronized cultures. High levels of ccrM transcription despite low levels of CtrA and increased transcription of ctrA P2 in the ctrA P1 mutant are two examples of robustness in the cell cycle. Thus, Caulobacter can adjust regulatory pathways to partially compensate for reduced and delayed CtrA accumulation in the ctrA P1 mutant.
    Full-text · Article · Jul 2012 · Microbiology
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    • "We have identified the Caulobacter crescentus essential genome to 8 bp resolution by performing ultrahigh-resolution transposon mutagenesis followed by high-throughput DNA sequencing to determine the transposon insertion sites. A notable feature of C. crescentus is that the regulatory events that control polar differentiation and cell-cycle progression are highly integrated, and they occur in a temporally restricted order (McAdams and Shapiro, 2011). Many components of the core regulatory circuit have been identified and simulation of the circuitry has been reported (Shen et al, 2008). "
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    ABSTRACT: The regulatory events that control polar differentiation and cell-cycle progression in the bacterium Caulobacter crescentus are highly integrated, and they have to occur in the proper order (McAdams and Shapiro, 2011). Components of the core regulatory circuit are largely known. Full discovery of its essential genome, including non-coding, regulatory and coding elements, is a prerequisite for understanding the complete regulatory network of this bacterial cell. We have identified all the essential coding and non-coding elements of the Caulobacter chromosome using a hyper-saturated transposon mutagenesis strategy that is scalable and can be readily extended to obtain rapid and accurate identification of the essential genome elements of any sequenced bacterial species at a resolution of a few base pairs. We engineered a Tn5 derivative transposon (Tn5Pxyl) that carries at one end an inducible outward pointing Pxyl promoter (Christen et al, 2010). We showed that this transposon construct inserts into the genome randomly where it can activate or disrupt transcription at the site of integration, depending on the insertion orientation. DNA from hundred of thousands of transposon insertion sites reading outward into flanking genomic regions was parallel PCR amplified and sequenced by Illumina paired-end sequencing to locate the insertion site in each mutant strain (Figure 1). A single sequencing run on DNA from a mutagenized cell population yielded 118 million raw sequencing reads. Of these, >90 million (>80%) read outward from the transposon element into adjacent genomic DNA regions and the insertion site could be mapped with single nucleotide resolution. This yielded the location and orientation of 428 735 independent transposon insertions in the 4-Mbp Caulobacter genome. Within non-coding sequences of the Caulobacter genome, we detected 130 non-disruptable DNA segments between 90 and 393 bp long in addition to all essential promoter elements. Among 27 previously identified and validated sRNAs (Landt et al, 2008), three were contained within non-disruptable DNA segments and another three were partially disruptable, that is, insertions caused a notable growth defect. Two additional small RNAs found to be essential are the transfer-messenger RNA (tmRNA) and the ribozyme RNAseP (Landt et al, 2008). In addition to the 8 non-disruptable sRNAs, 29 out of the 130 intergenic essential non-coding sequences contained non-redundant tRNA genes; duplicated tRNA genes were non-essential. We also identified two non-disruptable DNA segments within the chromosomal origin of replication. Thus, we resolved essential non-coding RNAs, tRNAs and essential replication elements within the origin region of the chromosome. An additional 90 non-disruptable small genome elements of currently unknown function were identified. Eighteen of these are conserved in at least one closely related species. Only 2 could encode a protein of over 50 amino acids. For each of the 3876 annotated open reading frames (ORFs), we analyzed the distribution, orientation, and genetic context of transposon insertions. There are 480 essential ORFs and 3240 non-essential ORFs. In addition, there were 156 ORFs that severely impacted fitness when mutated. The 8-bp resolution allowed a dissection of the essential and non-essential regions of the coding sequences. Sixty ORFs had transposon insertions within a significant portion of their 3′ region but lacked insertions in the essential 5′ coding region, allowing the identification of non-essential protein segments. For example, transposon insertions in the essential cell-cycle regulatory gene divL, a tyrosine kinase, showed that the last 204 C-terminal amino acids did not impact viability, confirming previous reports that the C-terminal ATPase domain of DivL is dispensable for viability (Reisinger et al, 2007; Iniesta et al, 2010). In addition, we found that 30 out of 480 (6.3%) of the essential ORFs appear to be shorter than the annotated ORF, suggesting that these are probably mis-annotated. Among the 480 ORFs essential for growth on rich media, there were 10 essential transcriptional regulatory proteins, including 5 previously identified cell-cycle regulators (McAdams and Shapiro, 2003; Holtzendorff et al, 2004; Collier and Shapiro, 2007; Gora et al, 2010; Tan et al, 2010) and 5 uncharacterized predicted transcription factors. In addition, two RNA polymerase sigma factors RpoH and RpoD, as well as the anti-sigma factor ChrR, which mitigates rpoE-dependent stress response under physiological growth conditions (Lourenco and Gomes, 2009), were also found to be essential. Thus, a set of 10 transcription factors, 2 RNA polymerase sigma factors and 1 anti-sigma factor are the core essential transcriptional regulators for growth on rich media. To further characterize the core components of the Caulobacter cell-cycle control network, we identified all essential regulatory sequences and operon transcripts. Altogether, the 480 essential protein-coding and 37 essential RNA-coding Caulobacter genes are organized into operons such that 402 individual promoter regions are sufficient to regulate their expression. Of these 402 essential promoters, the transcription start sites (TSSs) of 105 were previously identified (McGrath et al, 2007). The essential genome features are non-uniformly distributed on the Caulobacter genome and enriched near the origin and the terminus regions. In contrast, the chromosomal positions of the published E. coli essential coding sequences (Rocha, 2004) are preferentially located at either side of the origin (Figure 4A). This indicates that there are selective pressures on chromosomal positioning of some essential elements (Figure 4A). The strategy described in this report could be readily extended to quickly determine the essential genome for a large class of bacterial species.
    Full-text · Article · Aug 2011 · Molecular Systems Biology
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    ABSTRACT: Three novel LC-UV methods for the determination of pentamidine (PTMD) and two of its new analogs in rat plasma are described. The chromatographic conditions (wavelength, acetonitrile percentage in the mobile phase, internal standard) were optimized to have an efficient selectivity. A pre-step of extraction was simultaneously developed for each compound. For PTMD, a solid phase extraction (SPE) with Oasis(®) HLB cartridges was selected, while for the analogs we used protein precipitation with acetonitrile. SPE for PTMD gave excellent results in terms of extraction yield (99.7 ± 2.8) whereas the recoveries for the analogs were not so high but were reproducible as well (64.6 ± 2.6 and 36.8 ± 1.6 for analog 1 and 2, respectively). By means of a recent strategy based on accuracy profiles (β-expectation tolerance interval), the methods were successfully validated. β was fixed at 95% and the acceptability limits at ± 15% as recommended by the FDA. The method was successfully validated for PTMD (29.6-586.54 ng/mL), analog 1 (74.23-742.3 ng/mL) and analog 2 (178.12-890.6 ng/mL). The first concentration level tested was considered as the LLOQ (lower limit of quantification) for PTMD and analog 1 whereas for analog 2, the LLOQ was not the first level tested and was raised to 178.12 ng/mL.
    No preview · Article · Jan 2011 · Talanta
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