Mobile Intron Meets Magic Spot

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Science  18 Jul 2008:
Vol. 321, Issue 5887, pp. 319
DOI: 10.1126/science.321.5887.319d

Early on, introns were usually thought of as useless pieces of transcribed RNA that needed to be excised before the product RNA could become fully active. The later identification of intron-encoded species, such as microRNAs, has emphasized the utility of anything and everything that a cell carries around. In Lactococcus lactis, there is a mobile group II intron that consists of the catalytic LtrB RNA and the intron-encoded protein LtrA. The protein serves to stabilize the active conformation of the RNA, which splices itself out of transcripts, and also supplies a reverse transcriptase activity, which enables LtrB to insert itself at vulnerable sites in genomic DNA. One such site is the origin of replication locus (oriC) that in Escherichia coli is located at the ends of each rod-shaped cell.

Zhao et al. show that LtrA localizes to the poles as well and thus accounts for the preferential integration of LtrB at oriC. They also find, surprisingly, that LtrA binds to polyphosphate, a curious metabolite that increases under conditions of stress and is degraded by the enzyme that synthesizes ppGpp (magic spot); this interaction has the consequence of spreading LtrA and other nucleic acid-binding proteins throughout the cell when nutrients become scarce. Whether this diffusion of polar components is the long-sought function of polyphosphate remains to be determined. — GJC

PLoS Biol. 6, e150 (2008).

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