Rapid self-test of unprocessed viruses of SARS-CoV-2 and its variants in saliva by portable wireless graphene biosensor

We have developed a DNA aptamer-conjugated graphene field-effect transistor (GFET) biosensor platform to detect receptor-binding domain (RBD), nucleocapsid (N), and spike (S) proteins, as well as viral particles of original Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) coronavirus and its variants in saliva samples. The GFET biosensor is a label-free, rapid (≤20 min), ultrasensitive handheld wireless readout device. The limit of detection (LoD) and the limit of quantitation (LoQ) of the sensor are 1.28 and 3.89 plaque-forming units (PFU)/mL for S protein and 1.45 and 4.39 PFU/mL for N protein, respectively. Cognate spike proteins of major variants of concern (N501Y, D614G, Y453F, Omicron-B1.1.529) showed sensor response ≥40 mV from the control (aptamer alone) for fM to nM concentration range. The sensor response was significantly lower for viral particles and cognate proteins of Middle East Respiratory Syndrome (MERS) compared to SARS-CoV-2, indicating the specificity of the diagnostic platform for SARS-CoV-2 vs. MERS viral proteins. During the early phase of the pandemic, the GFET sensor response agreed with RT-PCR data for oral human samples, as determined by the negative percent agreement (NPA) and positive percent agreement (PPA). During the recent Delta/Omicron wave, the GFET sensor also reliably distinguished positive and negative clinical saliva samples. Although the sensitivity is lower during the later pandemic phase, the GFET-defined positivity rate is in statistically close alignment with the epidemiological population-scale data. Thus, the aptamer-based GFET biosensor has a high level of precision in clinically and epidemiologically significant SARS-CoV-2 variant detection. This universal pathogen-sensing platform is amenable for a broad range of public health applications and real-time environmental monitoring.

Direct DNA Methylation Profiling with an Electric Biosensor

DNA methylation is one of the principal epigenetic mechanisms that control gene expression in humans, and its profiling provides critical information about health and disease. Current profiling methods require chemical modification of bases followed by sequencing, which is expensive and time-consuming. Here, we report a direct and rapid determination of DNA methylation using an electric biosensor. The device consists of a DNA-tweezer probe integrated on a graphene field-effect transistor for label-free, highly sensitive, and specific methylation profiling. The device performance was evaluated with a target DNA that harbors a sequence of the methylguanine-DNA methyltransferase, a promoter of glioblastoma multiforme, a lethal brain tumor. The results show that we successfully profiled the methylated and nonmethylated forms at picomolar concentrations. Further, fluorescence kinetics and molecular dynamics simulations revealed that the position of the methylation site(s), their proximity, and accessibility to the toe-hold region of the tweezer probe are the primary determinants of the device performance.

Novel Bioinformatics Approach Identifies Transcriptional Profiles of Lineage-Specific Transposable Elements at Distinct Loci in the Human Dorsolateral Prefrontal Cortex

Expression of transposable elements (TE) is transiently activated during human preimplantation embryogenesis in a developmental stage- and cell type-specific manner and TE-mediated epigenetic regulation is intrinsically wired in developmental genetic networks in human embryos and embryonic stem cells. However, there are no systematic studies devoted to a comprehensive analysis of the TE transcriptome in human adult organs and tissues, including human neural tissues. To investigate TE expression in the human Dorsolateral Prefrontal Cortex (DLPFC), we developed and validated a straightforward analytical approach to chart quantitative genome-wide expression profiles of all annotated TE loci based on unambiguous mapping of discrete TE-encoded transcripts using a de novo assembly strategy. To initially evaluate the potential regulatory impact of DLPFC-expressed TE, we adopted a comparative evolutionary genomics approach across humans, primates, and rodents to document conservation patterns, lineage-specificity, and colocalizations with transcription factor binding sites mapped within primate- and human-specific TE. We identified 654,665 transcripts expressed from 477,507 distinct loci of different TE classes and families, the majority of which appear to have originated from primate-specific sequences. We discovered 4,687 human-specific and transcriptionally active TEs in DLPFC, of which the prominent majority (80.2%) appears spliced. Our analyses revealed significant associations of DLPFC-expressed TE with primate- and human-specific transcription factor binding sites, suggesting potential cross-talks of concordant regulatory functions. We identified 1,689 TEs differentially expressed in the DLPFC of Schizophrenia patients, a majority of which is located within introns of 1,137 protein-coding genes. Our findings imply that identified DLPFC-expressed TEs may affect human brain structures and functions following different evolutionary trajectories. On one side, hundreds of thousands of TEs maintained a remarkably high conservation for ∼8 My of primates’ evolution, suggesting that they are likely conveying evolutionary-constrained primate-specific regulatory functions. In parallel, thousands of transcriptionally active human-specific TE loci emerged more recently, suggesting that they could be relevant for human-specific behavioral or cognitive functions.

The evolution of Great Apes has shaped the functional enhancers' landscape in human embryonic stem cells

High-throughput functional assays of enhancer activity have recently enabled the genome-scale definition of molecular, structural, and biochemical features of these genomic regulatory regions. To infer the evolutionary origin of DNA sequences operating as functional enhancers in human embryonic stem cells (hESC), we examined the patterns of evolutionary conservation and divergence in the genome-wide functional enhancers' landscape of hESC. We show that a prominent majority (up to 94%) of DNA sequences identified in hESC as functional enhancers are conserved in humans and our closest evolutionary relatives, Chimpanzee and Bonobo. More than 91% of functional enhancers that are highly conserved in both Chimpanzee and Bonobo, are conserved among other Great Apes and >75% are conserved in the Rhesus genome. In striking contrast, <5% of DNA sequences operating in hESC as functional enhancers are conserved in rodents. Conserved in primates enhancers' sequences are complemented by 1619 sequences of enhancers that are specific to humans. Enhancers that harbor human-specific sequences appear enriched among the invariant enhancer module maintaining activity in different pluripotent states and these regions are associated with pluripotency- and embryonic-lineage-related genes. However, functional enhancers make up only a minority of all conserved in primates or human-specific transcription factor binding sites. Our analyses revealed that sequences that are conserved during ~8 million years of primate evolution dominate the genomic landscape of functional enhancers in both primed and naïve hESC. Collectively, these observations revealed thousands of evolutionarily conserved sequences that function as a core regulatory network in human embryonic stem cells which has recently undergone further extension after divergence of modern humans from our closest relatives, Chimpanzee and Bonobo.

DNA Nanotweezers and Graphene Transistor Enable Label-Free Genotyping

Electronic DNA-biosensor with a single nucleotide resolution capability is highly desirable for personalized medicine. However, existing DNA-biosensors, especially single nucleotide polymorphism (SNP) detection systems, have poor sensitivity and specificity and lack real-time wireless data transmission. DNA-tweezers with graphene field effect transistor (FET) are used for SNP detection and data are transmitted wirelessly for analysis. Picomolar sensitivity of quantitative SNP detection is achieved by observing changes in Dirac point shift and resistance change. The use of DNA-tweezers probe with high-quality graphene FET significantly improves analytical characteristics of SNP detection by enhancing the sensitivity more than 1000-fold in comparison to previous work. The electrical signal resulting from resistance changes triggered by DNA strand-displacement and related changes in the DNA geometry is recorded and transmitted remotely to personal electronics. Practical implementation of this enabling technology will provide cheaper, faster, and portable point-of-care molecular health status monitoring and diagnostic devices.

Single cell expression analysis of primate-specific retroviruses-derived HPAT lincRNAs in viable human blastocysts identifies embryonic cells co-expressing genetic markers of multiple lineages

Chromosome instability and aneuploidies occur very frequently in human embryos, impairing proper embryogenesis and leading to cell cycle arrest, loss of cell viability, and developmental failures in 50–80% of cleavage-stage embryos. This high frequency of cellular extinction events represents a significant experimental obstacle challenging analyses of individual cells isolated from human preimplantation embryos. We carried out single cell expression profiling of 241 individual cells recovered from 32 human embryos during the early and late stages of viable human blastocyst (VHB) differentiation. Classification of embryonic cells was performed solely based on expression patterns of human pluripotency-associated transcripts (HPAT), which represent a family of primate-specific transposable element-derived lincRNAs highly expressed in human embryonic stem cells and regulating nuclear reprogramming and pluripotency induction. We then validated our findings by analyzing transcriptomes of 1,708 individual cells recovered from more than 100 human embryos and 259 mouse cells from more than 40 mouse embryos at different stages of preimplantation embryogenesis. HPAT's expression-guided spatiotemporal reconstruction of human embryonic development inferred from single-cell expression analysis of VHB differentiation enabled identification of telomerase-positive embryonic cells co-expressing key pluripotency regulatory genes and genetic markers of three major lineages. Follow-up validation analyses confirmed the emergence in human embryos prior to lineage segregation of telomerase-positive cells co-expressing genetic markers of multiple lineages. Observations reported in this contribution support the hypothesis of a developmental pathway of creation embryonic lineages and extraembryonic tissues from telomerase-positive pre-lineage cells manifesting multi-lineage precursor phenotype.

Multifunctional stimuli responsive polymer-gated iron and gold-embedded silica nano golf balls: Nanoshuttles for targeted on-demand theranostics

Multi-functional nanoshuttles for remotely targeted and on-demand delivery of therapeutic molecules and imaging to defined tissues and organs hold great potentials in personalized medicine, including precise early diagnosis, efficient prevention and therapy without toxicity. Yet, in spite of 25 years of research, there are still no such shuttles available. To this end, we have designed magnetic and gold nanoparticles (NP)-embedded silica nanoshuttles (MGNSs) with nanopores on their surface. Fluorescently labeled Doxorubicin (DOX), a cancer drug, was loaded in the MGNSs as a payload. DOX loaded MGNSs were encapsulated in heat and pH sensitive polymer P(NIPAM-co-MAA) to enable controlled release of the payload. Magnetically-guided transport of MGNSs was examined in: (a) a glass capillary tube to simulate their delivery via blood vessels; and (b) porous hydrogels to simulate their transport in composite human tissues, including bone, cartilage, tendon, muscles and blood-brain barrier (BBB). The viscoelastic properties of hydrogels were examined by atomic force microscopy (AFM). Cellular uptake of DOX-loaded MGNSs and the subsequent pH and temperature-mediated release were demonstrated in differentiated human neurons derived from induced pluripotent stem cells (iPSCs) as well as epithelial HeLa cells. The presence of embedded iron and gold NPs in silica shells and polymer-coating are supported by SEM and TEM. Fluorescence spectroscopy and microscopy documented DOX loading in the MGNSs. Time-dependent transport of MGNSs guided by an external magnetic field was observed in both glass capillary tubes and in the porous hydrogel. AFM results affirmed that the stiffness of the hydrogels model the rigidity range from soft tissues to bone. pH and temperature-dependent drug release analysis showed stimuli responsive and gradual drug release. Cells' viability MTT assays showed that MGNSs are non-toxic. The cell death from on-demand DOX release was observed in both neurons and epithelial cells even though the drug release efficiency was higher in neurons. Therefore, development of smart nanoshuttles have significant translational potential for controlled delivery of theranostics' payloads and precisely guided transport in specified tissues and organs (for example, bone, cartilage, tendon, bone marrow, heart, lung, liver, kidney, and brain) for highly efficient personalized medicine applications.

In Situ Spatial Complementation of Aptamer-Mediated Recognition Enables Live-Cell Imaging of Native RNA Transcripts in Real Time

Direct cellular imaging of the localization and dynamics of biomolecules helps to understand their function and reveals novel mechanisms at the single-cell resolution. In contrast to routine fluorescent-protein-based protein imaging, technology for RNA imaging remains less well explored because of the lack of enabling technology. Herein, we report the development of an aptamer-initiated fluorescence complementation (AiFC) method for RNA imaging by engineering a green fluorescence protein (GFP)-mimicking turn-on RNA aptamer, Broccoli, into two split fragments that could tandemly bind to target mRNA. When genetically encoded in cells, endogenous mRNA molecules recruited Split-Broccoli and brought the two fragments into spatial proximity, which formed a fluorophore-binding site in situ and turned on fluorescence. Significantly, we demonstrated the use of AiFC for high-contrast and real-time imaging of endogenous RNA molecules in living mammalian cells. We envision wide application and practical utility of this enabling technology to in vivo single-cell visualization and mechanistic analysis of macromolecular interactions.

Dual-Functionalized Theranostic Nanocarriers

Nanocarriers with the ability to spatially organize chemically distinct multiple bioactive moieties will have wide combinatory therapeutic and diagnostic (theranostic) applications. We have designed dual-functionalized, 100 nm to 1 μm sized scalable nanocarriers comprising a silica golf ball with amine or quaternary ammonium functional groups located in its pits and hydroxyl groups located on its nonpit surface. These functionalized golf balls selectively captured 10-40 nm charged gold nanoparticles (GNPs) into their pits. The selective capture of GNPs in the golf ball pits is visualized by scanning electron microscopy. ζ potential measurements and analytical modeling indicate that the GNP capture involves its proximity to and the electric charge on the surface of the golf balls. Potential applications of these dual-functionalized carriers include distinct attachment of multiple agents for multifunctional theranostic applications, selective scavenging, and clearance of harmful substances.

Magnetically-responsive silica-gold nanobowls for targeted delivery and SERS-based sensing

Composite colloidal structures with multi-functional properties have wide applications in targeted delivery of therapeutics and imaging contrast molecules and high-throughput molecular bio-sensing. We have constructed a multifunctional composite magnetic nanobowl using the bottom-up approach on an asymmetric silica/polystyrene Janus template consisting of a silica shell around a partially exposed polystyrene core. The nanobowl consists of a silica bowl and a gold exterior shell with iron oxide magnetic nanoparticles sandwiched between the silica and gold shells. The nanobowls were characterized by electron microscopy, atomic force microscopy, magnetometry, vis-NIR and FTIR spectroscopy. Magnetically vectored transport of these nanobowls was ascertained by time-lapsed imaging of their flow in fluid through a porous hydrogel under a defined magnetic field. These magnetically-responsive nanobowls show distinct surface enhanced Raman spectroscopy (SERS) imaging capability. The PEGylated magnetically-responsive nanobowls show size-dependent cellular uptake in vitro.

DNA nano-carrier for repeatable capture and release of biomolecules

DNA can be manipulated to design nano-machines through specific sequence recognition. We report a switchable DNA carrier for repeatable capture and release of a single stranded DNA. The activity of the carrier was regulated by the interactions among a double-stranded actuator, single stranded target, fuel, and anti-fuel DNA strands. Inosine was used to maintain a stable triple-stranded complex when the actuator's conformation was switched between open (capture) and closed (release) configurations. Time lapse fluorescence measurements show repeatable capture and release of target strands. TEM images also show visible capture of target DNA strands when gold nanoparticles were attached to the DNA carrier and the target DNA strand. The carrier activity was controlled by length of toeholds, number of mismatches, and inosine substitutions. Significantly, unlike in previously published work that reported the devices functioned only when there is a perfect match between the interacting DNA strands, the present device works only when there are mismatches in the fuel strand and the best performance is achieved for 1-3 mismatches. The device was used to successfully capture and release gold nanoparticles when linked to the target single-stranded DNA. In general, this type of devices can be used for transport and delivery of theranostic molecules.

Asymmetric Colloidal Janus Particle Formation Is Core-Size-Dependent

Colloidal particles with asymmetric surface chemistry (Janus particles) have unique bifunctional properties. The size of these particles is an important determinant for their applications in diverse fields from drug delivery to chemical catalysis. The size of Janus particles, with a core surface coated with carboxylate and a partially encapsulating silica shell, depends upon several factors, including the core size and the concentration of carboxylate coating. The role of the carboxylate coating on the Janus particle size is well-understood; however, the role of the core size is not well defined. The role of the carboxylated polystyrene (cPS) core size on the cPS-silica Janus particle morphology (its size and shape) was examined by testing two different silica sizes and five different cPS core sizes. Results from electron microscopy (EM) and dynamic light scattering (DLS) analysis indicate that the composite cPS-silica particle acquires two distinct shapes: (i) when the size of the cPS core is much smaller than the non-cPS silica (b-SiO2) sphere, partially encapsulated Janus particles are formed, and (ii) when the cPS core is larger than or equal to the b-SiO2 sphere, a raspberry-like structure rather than a Janus particle is formed. The cPS-silica Janus particles of ∼100-500 nm size were obtained when the size of the cPS core was much smaller than the non-cPS silica (b-SiO2) sphere. These scalable nanoscale Janus particles will have wide application in a multifunctional delivery platform and catalysis.

Energetically biased DNA motor containing a thermodynamically stable partial strand displacement state

Current work in tuning DNA kinetics has focused on changing toehold lengths and DNA concentrations. However, kinetics can also be improved by enhancing the completion probability of the strand displacement process. Here, we execute this strategy by creating a toehold DNA motor device with the inclusion of a synthetic nucleotide, inosine, at selected sites. Furthermore, we found that the energetic bias can be tuned such that the device can stay in a stable partially displaced state. This work demonstrates the utility of energetic biases to change DNA strand displacement kinetics and introduces a complementary strategy to the existing designs.

Characterization of mammary cancer stem cells in the MMTV-PyMT mouse model

Breast cancer stem cells, the root of tumor growth, present challenges to investigate: Primary human breast cancer cells are difficult to establish in culture and inconsistently yield tumors after transplantation into immune-deficient recipient mice. Furthermore, there is limited characterization of mammary cancer stem cells in mice, the ideal model for the study of breast cancer. We herein describe a pre-clinical breast cancer stem cell model, based on the properties of cancer stem cells, derived from transgenic MMTV-PyMT mice. Using a defined set of cell surface markers to identify cancer stem cells by flow cytometry, at least four cell populations were recovered from primary mammary cancers. Only two of the four populations, one epithelial and one mesenchymal, were able to survive and proliferate in vitro. The epithelial population exhibited tumor initiation potential with as few as 10 cells injected into syngeneic immune-competent recipients. Tumors initiated from injected cell lines recapitulated the morphological and physiological components of the primary tumor. To highlight the stemness potential of the putative cancer stem cells, B lymphoma Mo-MLV insertion region 1 homolog (Bmi-1) expression was knocked down via shRNA targeting Bmi-1. Without Bmi-1 expression, putative cancer stem cells could no longer initiate tumors, but tumor initiation was rescued with the introduction of a Bmi-1 overexpression vector in the Bmi-1 knockdown cells. In conclusion, our data show that primary mammary cancers from MMTV-PyMT mice contain putative cancer stem cells that survive in culture and can be used to create a model for study of mammary cancer stem cells.

Direct and indirect contribution of bone marrow-derived cells to cancer

Stromal-epithelial interactions may control the growth and initiation of cancers. Here, we not only test the hypothesis that bone marrow-derived cells may effect development of cancers arising from other tissue cells by forming tumor stroma but also that sarcomas may arise by transformation of stem cells from the bone marrow and epithelial cancers may arise by transdifferentiation of bone marrow stem cells to epithelial cancers. Lethally irradiated female FVB/N mice were restored with bone marrow (BM) transplants from a male transgenic mouse carrying the polyoma middle T-oncoprotein under the control of the mouse mammary tumor virus promoter (MMTV-PyMT) and followed for development of lesions. All of 8 lethally irradiated female FVB/N recipient mice, restored with BM transplants from a male MMTV-PyMT transgenic mouse, developed Y-chromosome negative (Y-) cancers of various organs surrounded by Y+ stroma. One of the female FVB/N recipient mice also developed fibrosarcoma and 1, a diploid breast adenocarcinoma containing Y chromosomes. In contrast, only 1 of 12 control female mice restored with normal male BM developed a tumor (lymphoma) during the same time period. These results indicate not only that the transgenic BM-derived stromal cells may indirectly contribute to development of tumors in recipient mice but also that sarcomas may arise by transformation of BM stem cells and that breast cancers arise by transdifferentiation of BM stem cells, presumably by mesenchymal-epithelial transition.

Abstract 3344: Two different stem cell populations exist in breast cancer to control tumor initiation

The cancer stem cell theory states that a determinate number of cells within a tumor have unique, stem cell-like features to regenerate the entire tumor mass. However, a cancer stem cell may need the right microenvironment to have its greatest tumor initiation potential. We have documented the presence of two stem cell populations within a tumor to yield consistent and most aggressive recapitulation of tumor initiation in vivo. To isolate cancer stem cells, we used the MMTV-PyMT transgenic model of breast cancer in mice to generate stable cell lines from spontaneous breast tumors. Stable in vitro cultures of FACS analyzed cancer stem cells and tumor-residing mesenchymal stem cells were established and tested for tumor initiating ability using two methods of implantation: subcutaneous (s.c.) injection and cleared mammary fat pad (CMFP) injection. To determine the limit of tumor initiation, a highly malignant cancer stem cell culture (419) was injected s.c. This cell culture produced tumors with 1×104, 1×103, and 1×102 cells with 100%, 93%, and 33% tumorigenicity, respectively. In comparison, 100% tumorigenicity was obtained with 1×102 419 cells implanted in the CMFP, a significant increase compared to s.c. injection (Chi-squared, p&lt;0.001). S.c. injection of a less malignant cancer stem cell culture (145) produced tumors less efficiently, with 100% tumorigenicity with 1×104 cells but 13% tumorigenicity with 1×103 cells. To increase tumorigenicity at low cell numbers, cancer stem cell cultures were combined with tumor-residing mesenchymal stem cells just prior to injection. Cancer stem cell cultures were s.c. injected with 1×105 tumor-residing mesenchymal stem cells (383) at the tumor limiting dose, 1×101 for 419 and 1×103 for 145; tumorigenicity increased 160% in the highly malignant cells (419) and 133% in the less malignant cells (145). The tumor-residing mesenchymal stem cells increased tumorigenicity significantly compared to cancer stem cell injection alone (Chi-squared, p&lt;0.002). Control experiments using non-tumorigenic mesenchymal cells did not produce any increase in tumorigenicity. Genome-wide expression profiling experiments were carried out to demonstrate the clinical relevance of the varying malignancy settings in the cancer stem cell and tumor-residing mesenchymal stem cell cultures. Our results suggest that the cellular composition of the local microenvironment within the breast tumors is the major determinant of efficiency of tumor initiation. The addition of tumor-residing mesenchymal stem cells markedly increases the tumor initiation potential of breast cancer stem cells.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3344.

Small-molecule antagonists of apoptosis suppressor XIAP exhibit broad antitumor activity

Apoptosis resistance commonly occurs in cancers, preventing activation of Caspase family cell death proteases. XIAP is an endogenous inhibitor of Caspases overexpressed in many cancers. We developed an enzyme derepression assay, based on overcoming XIAP-mediated suppression of Caspase-3, and screened mixture-based combinatorial chemical libraries for compounds that reversed XIAP-mediated inhibition of Caspase-3, identifying a class of polyphenylureas with XIAP-inhibitory activity. These compounds, but not inactive structural analogs, stimulated increases in Caspase activity, directly induced apoptosis of many types of tumor cell lines in culture, and sensitized cancer cells to chemotherapeutic drugs. Active compounds also suppressed growth of established tumors in xenograft models in mice, while displaying little toxicity to normal tissues. These findings validate IAPs as targets for cancer drug discovery.

Identification of peptide sequences that bind the Thomsen-Friedenreich cancer-associated glycoantigen from bacteriophage peptide display libraries

The goal of this study was to determine if polypeptides that bind specifically to the carcinoma-associated Thomsen-Friedenreich (T) antigen could be isolated from a random peptide bacteriophage display library. T antigen is a carbohydrate antigen that is exposed and immunoreactive on the surfaces of most primary carcinomas and their metastases, while it is masked on normal cells. Tumor-specific surface carbohydrates are often used as markers of cell differentiation and play a role in cell aggregation, which is an important step in the metastatic process. Therefore, peptides that bind and mask T antigen may yield useful carbohydrate-specific probes and provide insight into carbohydrate-mediated tumor-cell aggregation. A 15-amino acid random peptide bacteriophage display library was screened for polypeptides that exhibited high specificity to two glycoproteins which display T antigen on their surfaces. The results suggest that synthetic peptides identified from the bacteriophage display library have high affinities (Kd approximately 1 microM) and specificities for proteins and human tumor cells which present T antigen. Thus, random bacteriophage peptide display libraries may be a rich source of sequences that bind to carbohydrate antigen structures.

Role of rpoS (katF) in oxyR-independent regulation of hydroperoxidase I in Escherichia coli

We present evidence showing that rpoS (katF) is a regulator of katG gene transcription in an oxyR-independent manner. Mutation of the rpoS gene in several different Escherichia coli strains caused a significant reduction in catalase HPI activity. In rpoS-delta oxyR double mutants, the level of HPI was considerably lower compared to the delta oxyR parent strain, and was restored when transformed with an rpoS+ plasmid. Overproduction of HPI in oxyR- suppressor strains was greatly diminished after inactivation of the rpoS gene and was accompanied by a substantial increase in sensitivity to menadione. Beta-galactosidase expression from a katG::lacZ promoter was lower in rpoS strains compared to rpoS+ isogenic parents. Several delta oxyR strains had detectable levels of katG transcription that was significantly diminished after rpoS gene inactivation.