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If experiments like those of Luria and Delbruck on E. coli and T1 phage are the main source of our confidence in the mutation-immunity model, is it then highly unlikely that there are other types of bacteria for which an encounter with a phage is the mutagenic (or immunity-conferring) event?

Are the two possibilities in principle mutually exclusive?

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No. There was a recent surprising paper on a bacterial adaptive immune system. I cannot find it, unfortunately. –  rwst Sep 7 '12 at 7:16
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No. Surprisingly, there is an adaptive immune system in prokaryotes. This is still widely unknown. The newest review is

S. Al-Attar, E. R. Westra et al: Clustered regularly interspaced short palindromic repeats (CRISPRs): the hallmark of an ingenious antiviral defense mechanism in prokaryotes. In: Biological chemistry. 392, 4, April 2011, 277–289. doi:10.1515/BC.2011.042. PMID 21294681. (Review).

Cited from abstract:

Many prokaryotes contain the recently discovered defense system against mobile genetic elements. This defense system contains a unique type of repetitive DNA stretches, termed Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs). CRISPRs consist of identical repeated DNA sequences (repeats), interspaced by highly variable sequences referred to as spacers. The spacers originate from either phages or plasmids and comprise the prokaryotes' 'immunological memory'. CRISPR-associated (cas) genes encode conserved proteins that together with CRISPRs make-up the CRISPR/Cas system, responsible for defending the prokaryotic cell against invaders.

and

An application of the CRISPR/Cas system is the immunization of industry-relevant prokaryotes (or eukaryotes) against mobile-genetic invasion. In addition, the high variability of the CRISPR spacer content can be exploited for phylogenetic and evolutionary studies. Despite impressive progress during the last couple of years, the elucidation of several fundamental details will be a major challenge in future research.

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Nice find and surprising (to me). Thanks. –  daniel Sep 7 '12 at 11:05
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