Biology subjects

Fritts, R. K.

Publications and source records attributed to Fritts, R. K..

4 recordsLinked to original sources

bioRxiv2025

Microbial cross-feeding is stabilized when a dependent mutant is segregated from its independent ancestor

Microbial gene loss is hypothesized to be beneficial when gene function is costly, and the gene product can be replaced via cross-feeding from a neighbor. However, cross-fed metabolites are often only available at low concentrations, limiting the growth rates of gene-loss mutants that are dependent on those metabolites. Here we define conditions that support a loss of function mutant in a three-member bacterial community of: (i) N2-utilizing Rhodopseudomonas palustris as an NH4+ -excreting producer, (ii) N2-utilizing Vibrio natriegens as the ancestor, and (iii) a V. natriegens N2-utilizaton mutant that is dependent on the producer for NH4+. Using experimental and simulated cocultures, we found that the ancestor outcompeted the mutant due to low NH4+ availability under uniform conditions where both V. natriegens strains have equal access to nutrients. However, spatial structuring that separated the mutant from the ancestor, while maintaining access to NH4+ from the producer, allowed the mutant to avoid extinction. Counter to predictions, mutant enrichment under spatially structured conditions did not require a growth rate advantage from gene loss and the mutant coexisted with its ancestor. Thus, cross-feeding can originate from loss-of-function mutations that are otherwise detrimental, provided that the mutant can segregate from a competitive ancestor. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=118 SRC="FIGDIR/small/634601v1_ufig1.gif" ALT="Figure 1"> View larger version (25K): org.highwire.dtl.DTLVardef@1140808org.highwire.dtl.DTLVardef@1b72e8org.highwire.dtl.DTLVardef@14f70borg.highwire.dtl.DTLVardef@6707bb_HPS_FORMAT_FIGEXP M_FIG C_FIG

microbiology↗

bioRxiv2023

The transcriptomic and proteomic ramifications of segmental amplification

Gene amplification can drive adaptation by rapidly increasing the cellular dosage of critical gene products. Segmental amplifications often encompass large genomic regions surrounding the gene(s) under selection for higher dosage. Overexpression of co-amplified neighboring genes imposes a substantial metabolic burden. While compensatory mutations can decrease inappropriate overexpression of co-amplified genes, it takes time for such mutations to arise. The extent to which intrinsic regulatory mechanisms modulate expression of co-amplified genes in the immediate aftermath of segmental amplification is largely unknown. To address the collateral effects of segmental amplification, we evolved replicate cultures of an Escherichia coli mutant under conditions that select for higher dosage of an inefficient enzyme whose weak activity limits growth rate. Segmental amplifications encompassing the gene encoding the weak-link enzyme arose in all populations. Amplicons ranged in size (9 to 125 kb) and copy number (2 to 12 copies). We performed RNA-seq and label-free proteomics to quantify expression of amplified genes present at 2, 6, and 12 copies. mRNA expression generally scales with gene copy number, but protein expression scales less well with both gene copy number and mRNA expression. We characterize the molecular mechanisms underlying discrepancies between gene copy number and expression for several cases. We also show that segmental amplifications can have system-wide consequences by indirectly altering expression of non-amplified genes. Our findings indicate that the fitness benefit derived from segmental amplification depends on the combined effects of amplification size, gene content, and copy number as well as collateral effects on non-amplified genes. Significance StatementGene amplification frequently drives rapid adaptation when organisms are challenged by harsh conditions. However, gene amplification rarely amplifies only the gene(s) under selection for higher dosage. Segmental amplifications often include many co-amplified neighbor genes. Because segmental amplifications can reach very high copy number, overexpression of co-amplified genes can impose an energetic burden and perturb physiology, yet this aspect of gene amplification has received little attention. Here, we performed transcriptomic and proteomic analysis of laboratory-evolved Escherichia coli strains containing large, high-copy-number amplifications. We found that mRNA levels, but often not protein levels, scale well with gene copy number. We identified amplified genes that exhibit discrepancies between gene copy number and mRNA/protein expression and examine the molecular mechanisms underlying these discrepancies.

evolutionary biology↗

bioRxiv2019

Enhanced nutrient uptake is sufficient to drive emergent cross-feeding between bacteria

Interactive microbial communities are ubiquitous, influencing biogeochemical cycles and host health. One widespread interaction is nutrient exchange, or cross-feeding, wherein metabolites are transferred between microbes. Some cross-fed metabolites, such as vitamins, amino acids, and ammonium (NH4+), are communally valuable and impose a cost on the producer. The mechanisms that enforce cross-feeding of communally valuable metabolites are not fully understood. Previously we engineered mutualistic cross-feeding between N2-fixing Rhodopseudomonas palustris and fermentative Escherichia coli. Engineered R. palustris excreted essential nitrogen as NH4+ to E. coli while E. coli excreted essential carbon as fermentation products to R. palustris. Here, we enriched for nascent cross-feeding in cocultures with wild-type R. palustris, not known to excrete NH4+. Emergent NH4+ cross-feeding was driven by adaptation of E. coli alone. A missense mutation in E. coli NtrC, a regulator of nitrogen scavenging, resulted in constitutive activation of an NH4+ transporter. This activity likely allowed E. coli to subsist on the small amount of leaked NH4+ and better reciprocate through elevated excretion of organic acids from a larger E. coli population. Our results indicate that enhanced nutrient uptake by recipients, rather than increased excretion by producers, is an underappreciated yet possibly prevalent mechanism by which cross-feeding can emerge.

microbiology↗

bioRxiv2016

A Rhizobiales-specific unipolar polysaccharide adhesin contributes to Rhodopseudomonas palustris biofilm formation across diverse photoheterotrophic conditions

Bacteria predominantly exist as members of surfaced-attached communities known as biofilms. Many bacterial species initiate biofilms and adhere to each other using cell surface adhesins. This is the case for numerous ecologically diverse -proteobacteria, which use polar exopolysaccharide adhesins for cell-cell adhesion and surface attachment. Here, we show that Rhodopseudomonas palustris, a metabolically versatile member of the -proteobacterial order Rhizobiales, encodes a functional unipolar polysaccharide (UPP) biosynthesis gene cluster. Deletion of genes predicted to be critical for UPP biosynthesis and export abolished UPP production. We also found that R. palustris uses UPP to mediate biofilm formation across diverse photoheterotrophic growth conditions, wherein light and organic substrates are used to support growth. However, UPP was less important for biofilm formation during photoautotrophy, where light and CO2 support growth, and during aerobic respiration with organic compounds. Expanding our analysis beyond R. palustris, we examined the phylogenetic distribution and genomic organization of UPP gene clusters among Rhizobiales species that inhabit diverse niches. Our analysis suggests that UPP is a conserved ancestral trait of the Rhizobiales but that it has been independently lost multiple times during the evolution of this clade, twice coinciding with adaptation to intracellular lifestyles within animal hosts.\n\nIMPORTANCEBacteria are ubiquitously found as surface-attached communities and cellular aggregates in nature. Here, we address how bacterial adhesion is coordinated in response to diverse environments using two complementary approaches. First, we examined how Rhodopseudomonas palustris, one of the most metabolically versatile organisms ever described, varies its adhesion to surfaces in response to different environmental conditions. We identified 2 critical genes for the production of a unipolar polysaccharide (UPP) and showed that UPP is important for adhesion when light and organic substrates are used for growth. Looking beyond R. palustris, we performed the most comprehensive survey to date on the conservation of UPP biosynthesis genes among a group of closely related bacteria that occupy diverse niches. Our findings suggest that UPP is important for free-living and plant-associated lifestyles but dispensable for animal pathogens. Additionally, we propose guidelines for classifying the adhesins produced by various -proteobacteria, facilitating future functional and comparative studies.

microbiology↗