Evolutionary dominance of holin lysis systems derives from superior genetic malleability.
For the microviruses and the leviviruses, bacteriophages with small single-stranded genomes, host lysis is accomplished by expression of a single gene that encodes an inhibitor of cell wall synthesis. In contrast, phages with double-stranded DNA genomes use a more complex system involving, at minimum, an endolysin, which degrades peptidoglycan, and a holin, which permeabilizes the membrane in a temporally programmed manner. To explore the basis of this difference, a chimera was created in which lysis gene E of the microvirus phiX174 replaced the entire lysis cassette of phage lambda, which includes the holin gene S and the endolysin gene R. The chimeric phage was viable but more variability was observed both in the distribution of plaque sizes and in the burst sizes of single cells, compared to the isogenic S(+) parent. Using different alleles of E, it was found the average burst size increased with the duration of the latent period, just as observed with S alleles with different lysis times. Moreover, within a set of missense E alleles, it was found that variability in lysis timing was limited and almost exclusively derived from changes in the level of E accumulation. By contrast, missense mutations in S resulted in a wide variation in lysis times that was not correlated with levels of accumulation. We suggest that the properties of greater phenotypic plasticity and lesser phenotypic variation make the function of holin proteins more genetically malleable, facilitating rapid adaptation towards a lysis time that would be optimal for changed host and environmental conditions. The inferior malleability of single-gene systems like E would restrict their occurrence to phages in which coding capacity is the overriding evolutionary constraint.
Department of Biochemistry and Biophysics, Texas A&M University, 2128 TAMU, College Station, TX 77843-2128, USA.
Yi
Y
Zheng
Department of Biochemistry and Biophysics, Texas A&M University, 2128 TAMU, College Station, TX 77843-2128, USA.
Douglas K
DK
Struck
Department of Biochemistry and Biophysics, Texas A&M University, 2128 TAMU, College Station, TX 77843-2128, USA.
Chelsey A
CA
Dankenbring
Department of Biochemistry and Biophysics, Texas A&M University, 2128 TAMU, College Station, TX 77843-2128, USA.
Ry
R
Young
D000483:Alleles; D000595:Amino Acid Sequence; D010582:Bacteriophage lambda / Q000235:genetics / Q000254:growth & development; D010584:Bacteriophage phi X 174 / Q000235:genetics; D005075:Biological Evolution; D004926:Escherichia coli / Q000821:virology; D008242:Lysogeny / Q000235:genetics*; D020125:Mutation, Missense; D010641:Phenotype; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D013997:Time Factors; D010948:Viral Plaque Assay; D014764:Viral Proteins / Q000235:genetics / Q000378:metabolism*
C029555:E protein, bacteriophage X174; D011993:Recombinant Fusion Proteins; C108624:S holin, bacteriophage lambda; D014764:Viral Proteins