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J. Bacteriol., 12 1996, 6921-6929, Vol 178, No. 23
LR Garcia and IJ Molineux
Penetration of wild-type T7 DNA into the host cell occurs in two steps. The
phage particle ejects a few hundred base pairs of the left end of the
genome into the host. Translocation of the remainder of the DNA is then
coupled to transcription. In a normal infection, transcription- coupled
translocation of wild-type T7 DNA is initiated at the major A1, A2, and A3
promoters for Escherichia coli RNA polymerase. At 37 degrees C, various
deletion mutants lacking these three promoters grow at the same efficiency
as wild-type T7 because the minor B promoter is efficiently transferred
from the phage head into the cell. As the temperature of the phage
infection decreases, the latent periods of (A1, A2, A3)- phages increase
relative to that of wild-type T7; nevertheless, (A1, A2, A3)- phages have
normal plating efficiencies at reduced temperatures. Lengthening of the
latent period at low temperatures is due to a delay in transferring the
complete (A1, A2, A3)- genome into the host cell. The (A1, A2, A3)- phages
eject the leading end of their genome into the host, but at low
temperature, insufficient DNA is transferred into the cell to allow RNA
polymerase immediate access the B promoter. However, by an inefficient
transcription-independent process, the B promoter eventually translocates
into the cell. Mutant derivatives of (A1, A2, A3)- phages that have growth
profiles at low temperatures similar to that of wild- type T7 have been
isolated. The mutations allow both (A1, A2, A3)- and (A1, A2, A3)+ phages
to translocate their entire genomes into the cell by a
transcription-independent mechanism. The mutations are located in gene 16,
a gene that encodes a component of the internal virion core. We postulate
that gp16 is directly involved with the process of DNA translocation from
the virion into the cell.
Copyright © 1996, American Society for Microbiology
Transcription-independent DNA translocation of bacteriophage T7 DNA into Escherichia coli
Department of Microbiology, University of Texas, Austin 78712-1095, USA.
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