DNA packaging by lambda-like bacteriophages: mutations broadening the packaging specificity of terminase, the lambda-packaging enzyme.
The DNA-packaging specificities of phages lambda and 21 depend on the specific DNA interactions of the small terminase subunits, which have support helix-turn-recognition helix-wing DNA-binding motifs. lambda-Terminase with the recognition helix of 21 preferentially packages 21 DNA. This chimeric terminase's ability to package lambdaDNA is reduced approximately 20-fold. Phage lambda with the chimeric terminase is unable to form plaques, but pseudorevertants are readily obtained. Some pseudorevertants have trans-acting suppressors that change codons of the recognition helix. Some of these codons appear to remove an unfavorable base-pair contact; others appear to create a novel nonspecific DNA contact. Helper-packaging experiments show that these mutant terminases have lost the ability to discriminate between lambda and 21 during DNA packaging. Two cis-acting suppressors affect cosB, the small subunit's DNA-binding site. Each changes a cosB(lambda)-specific base pair to a cosB(21)-specific base pair. These cosB suppressors cause enhanced DNA packaging by 21-specific terminase and reduce packaging by lambda-terminase. Both the cognate support helix and turn are required for strong packaging discrimination. The wing does not contribute to cosB specificity. Evolution of packaging specificity is discussed, including a model in which lambda- and 21-packaging specificities diverged from a common ancestor phage with broad packaging specificity.
Department of Microbiology, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, Iowa 52242, USA. michael-feiss@uiowa.edu
Michael
M
Feiss
Erin
E
Reynolds
Morgan
M
Schrock
Jean
J
Sippy
D000595:Amino Acid Sequence; D010582:Bacteriophage lambda / Q000201:enzymology* / Q000235:genetics*; D001483:Base Sequence; D042003:DNA Packaging; D004274:DNA, Recombinant / Q000235:genetics; D004279:DNA, Viral / Q000235:genetics / Q000378:metabolism*; D004706:Endodeoxyribonucleases / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D005818:Genetic Engineering; D005838:Genotype; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D017433:Protein Structure, Secondary; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013045:Species Specificity; D013379:Substrate Specificity; D013489:Suppression, Genetic
D004274:DNA, Recombinant; D004279:DNA, Viral; D021122:Protein Subunits; D011993:Recombinant Fusion Proteins; D004706:Endodeoxyribonucleases; C030894:terminase