Regulation of icosahedral virion capsid size by the in vivo activity of a cloned gene product.
Determination of icosahedral virion capsid size can be directly studied during helper-dependent lytic development of satellite P4 because the assembly pathway specified by the P2 helper virus is altered to yield smaller-sized capsids. Size determination (sid) mutations identify a P4-encoded function regulating this process. To determine whether the sid gene product is necessary and sufficient to redirect the assembly pathway, we (i) cloned the sid structural gene in a plasmid vector (pMA30) under the control of an inducible promoter and (ii) constructed a packaging substrate (pMA1), a P4 genome-sized plasmid containing only that region of P4, the cos site, necessary for encapsidation. Superinfection by P2 of a host carrying pMA30 under induced conditions resulted in a shift from large to small capsid production. P2 superinfection of a host carrying the cos plasmid pMA1 plus pMA30 under induced conditions yielded pMA1-transducing particles of P4 capsid size. These cloning-based analyses directly demonstrate that sid protein is the only P4 gene product required for small-capsid size determination. In the absence of the P2 O gene product no capsids of any size are assembled during solo infection by P2. Nevertheless, P2 Oam mutant superinfection of a host carrying pMA1 and pMA30 under induced conditions yielded small P4-sized transducing particles. We therefore propose that (i) the sid gene product competes with the O gene product to determine the assembly of small vs. large capsid sizes and (ii) both gene products probably function as temporary scaffolding proteins.
Agarwal
M
M
Section of Molecular Genetics and Epidemiology, Maxwell Finland Laboratory for Infectious Diseases, Boston University School of Public Health, MA 02118.
Arthur
M
M
Arbeit
R D
RD
Goldstein
R
R
D001483:Base Sequence; D002213:Capsid / Q000235:genetics* / Q000302:isolation & purification / Q000648:ultrastructure; D003001:Cloning, Molecular / Q000379:methods; D004926:Escherichia coli / Q000235:genetics; D015967:Gene Expression Regulation, Viral; D005814:Genes, Viral; D005816:Genetic Complementation Test; D008969:Molecular Sequence Data; D015345:Oligonucleotide Probes; D010957:Plasmids; D015183:Restriction Mapping; D012526:Satellite Viruses / Q000235:genetics*; D014771:Virion / Q000235:genetics*
D015345:Oligonucleotide Probes