Xia's Ontogeny & Phylogeny III

D. Z. Skinner dzolek at MATT.KSU.KSU.EDU
Fri Apr 22 18:45:52 EST 1994

Xia wrote:
>It turns out that the phage does make decisions. Once it is inside the
>bacterium, it checks, directly, whether the medium is rich (i.e., whether
>the bacterial colony is rapidly replicating). If the bacteria/phage ratio 
>is large (i.e., there are many hosts available), the lytic developmental 
>pathway is chosen. The phage rapidly replicates, lyses the host, and invade 
>other hosts. On the other hand, if the bacteria/phage ratio is small, the 
>lysogenic pathway is taken. The phage squeezes itself into the cozy home of
>the host genome and awaits for future opportunities of replication. In this
>state the phage is called a prophage.

>But a bacterium, being vulnerable itself, is by no means a safe heaven for
>the prophage. There are many factors that can damage bacterial DNA and 
>destroy the cozy home of the prophage. For this reason, the prophage has to 
>collect and analyse information of its host, and make decisions based on the
>analysis. Almost as soon as you expose the host bacteria to physical or
>chemical factors that damages the bacterial DNA, the prophage will rapidly
>decide to desert its host. It will suspend the expression of genes that
>maintain the lysogenic phase, cut itself out of its host genome, initiate
>the lytic developmental pathway, replicates and lyses its already fatally
>injured host.

Whoa! Hold up!  Aren't you being a bit teleological here? How can a phage,
with only ~48K basepairs of DNA, no systems, no nerves, certainly no brain,
make a DECISION??? How can it collect and analyze information from its
host?  Isn't what really happens, with the phage DNA sequestered in the E.
coli chromosome and, at least to the transcription machinery,
indistinguishable from it, a result of the HOST preferentially turning on
certain phage genes?  It seems to me the phage is less a diabolical
"creature" and more an evolutionarily sound mechanism of selecting against
replication of compromised bacterial genomes!

D. Z. Skinner
USDA-ARS and Agronomy       e-mail: dzolek at pop.ksu.edu  -or- dzs at rust.pp.ksu.edu
Throckmorton Hall                       bitmail: dzolek at ksuvm.ksu.edu
Kansas State University             Voice: (913) 532-7247                  
Manhattan, KS 66506-5501       Fax: (913)532-5692

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