mRNA stabilizing signals encoded in the genome of the bacteriophage phi x174

A computational model for bacteriophage ϕX174 gene expression

Bacteriophage ϕX174 has been widely used as a model organism to study fundamental processes in molecular biology. However, several aspects of ϕX174 gene regulation are not fully resolved. Here we construct a computational model for ϕX174 and use the model to study gene regulation during the phage infection cycle. We estimate the relative strengths of transcription regulatory elements (promoters and terminators) by fitting the model to transcriptomics data. We show that the specific arrangement of a promoter followed immediately by a terminator, which occurs naturally in the ϕX174 genome, poses a parameter identifiability problem for the model, since the activity of one element can be partially compensated for by the other. We also simulate ϕX174 gene expression with two additional, putative transcription regulatory elements that have been proposed in prior studies. We find that the activities of these putative elements are estimated to be weak, and that variation in ϕX174 transcript abundances can be adequately explained without them. Overall, our work demonstrates that ϕX174 gene regulation is well described by the canonical set of promoters and terminators widely used in the literature.

The Kinetic and Thermodynamic Aftermath of Horizontal Gene Transfer Governs Evolutionary Recovery

Shared host cells can serve as melting pots for viral genomes, giving many phylogenies a web-like appearance due to horizontal gene transfer. However, not all virus families exhibit web-like phylogenies. Microviruses form three distinct clades, represented by φX174, G4, and α3. Here, we investigate protein-based barriers to horizontal gene transfer between clades. We transferred gene G, which encodes a structural protein, between φX174 and G4, and monitored the evolutionary recovery of the resulting chimeras. In both cases, particle assembly was the major barrier after gene transfer. The G4φXG chimera displayed a temperature-sensitive assembly defect that could easily be corrected through single mutations that promote productive assembly. Gene transfer in the other direction was more problematic. The initial φXG4G chimera required an exogenous supply of both the φX174 major spike G and DNA pilot H proteins. Elevated DNA pilot protein levels may be required to compensate for off-pathway reactions that may have become thermodynamically and/or kinetically favored when the foreign spike protein was present. After three targeted genetic selections, the foreign spike protein was productively integrated into the φX174 background. The first adaption involved a global decrease in gene expression. This was followed by modifications affecting key protein-protein interactions that govern assembly. Finally, gene expression was re-elevated. Although the first selection suppresses nonproductive reactions, subsequent selections promote productive assembly and ultimately viability. However, viable chimeric strains exhibited reduced fitness compared with wild-type. This chimera's path to recovery may partially explain how unusual recombinant viruses could persist long enough to naturally emerge.

Differential transcription of bacteriophage φX174 genes at 37 °C and 42 °C

To investigate how high temperature affects viral transcription, the absolute amounts of mRNA for six bacteriophage φX174 genes were compared at 37°C and 42°C using Q-PCR. At 37°C, mRNA levels for all genes were consistent with previous studies, but at 42°C mRNA levels for four genes were significantly different from levels at 37°C. Transcript levels were higher for genes B and D; the promoter before gene B appears to be up-regulated at high temperature. Levels for genes F and G were reduced at high temperature, possibly due to increased efficiency of the transcription termination signal immediately upstream of gene F. These functional changes in φX174 gene regulation at high temperature have not been described previously. Studies of phage evolution at high temperatures indicate that this difference in transcript levels is subject to adaptation.

Positive selection at high temperature reduces gene transcription in the bacteriophage ϕX174

Background

Gene regulation plays a central role in the adaptation of organisms to their environments. There are many molecular components to gene regulation, and it is often difficult to determine both the genetic basis of adaptation and the evolutionary forces that influence regulation. In multiple evolution experiments with the bacteriophage ϕX174, adaptive substitutions in cis-acting regulatory sequences sweep through the phage population as the result of strong positive selection at high temperatures that are non-permissive for laboratory-adapted phage. For one cis-regulatory region, we investigate the individual effects of four adaptive substitutions on transcript levels and fitness for phage growing on three hosts at two temperatures.

Results

The effect of the four individual substitutions on transcript levels is to down-regulate gene expression, regardless of temperature or host. To ascertain the conditions under which these substitutions are adaptive, fitness was measured by a variety of methods for several bacterial hosts growing at two temperatures, the control temperature of 37°C and the selective temperature of 42°C. Time to lysis and doublings per hour indicate that the four substitutions individually improve fitness over the ancestral strain at high temperature independent of the bacterial host in which the fitness was measured. Competition assays between the ancestral strain and either of two mutant strains indicate that both mutants out-compete the ancestor at high temperature, but the relative frequencies of each phage remain the same at the control temperature.

Conclusions

Our results strongly suggest that gene transcription plays an important role in influencing fitness in the bacteriophage ϕX174, and different point mutations in a single cis-regulatory region provided the genetic basis for this role in adaptation to high temperature. We speculate that the adaptive nature of these substitutions is due to the physiology of the host at high temperature or the need to maintain particular ratios of phage proteins during capsid assembly. Our investigation of regulatory evolution contributes to interpreting genome-level assessments of regulatory variation, as well as to understanding the molecular basis of adaptation.

Genetic analyses of putative conformation switching and cross-species inhibitory domains in Microviridae external scaffolding proteins

Putative conformational switching and inhibitory regions in the Microviridae external scaffolding protein were investigated. Substitutions for glycine 61, hypothesized to promote a postdimerization conformational switch, have dominant lethal phenotypes. In previous studies, chimeric alpha3/phiX174 proteins for structures alpha-helix 1 and loop 6/alpha-helix 7 inhibited phiX174 morphogenesis when expressed from high copy number plasmids. To determine if inhibition was due to overexpression, chimeric genes were constructed into the phiX174 genome. In coinfections with wild-type, protein ratios would be 1:1. The helix 1 chimera has a recessive lethal phenotype; thus, overexpression confers inhibition. In single infections, the mutant cannot form procapsids, suggesting that helix 1 mediates the initial recognition of structural proteins. The lethal chimeric helix 7 protein has a dominant phenotype. Alone, the mutant forms defective procapsids, suggesting a later morphogenetic defect. The results of second-site genetic analyses indicate that the capsid-external scaffolding protein interface is larger than revealed in the crystal structure.