Publication Date: 2010 Mar 11 PMID: 20227418Authors: Ionescu, D. - Voss, B. - Oren, A. - Hess, W.
R. - Muro-Pastor, A. M.Journal: J Mol BiolIn response to nitrogen deficiency some cyanobacteria
develop heterocysts, a terminally differentiated cell type, specialized for the fixation of
atmospheric nitrogen. In Nostocales this differentiation process is controlled by two major
regulators, NtcA and HetR, but additional unknown factors are likely to be involved as well. In the
context of a genome-wide search for potential non-coding RNAs, we identified an array of 12 tandem
repeats that is transcribed in high amounts when cells enter conditions that trigger cell
differentiation and switch to nitrogen fixation. The main accumulating transcript, which we suggest
designating NsiR1, has properties similar to regulatory non-coding RNAs. In Anabaena sp. PCC 7120,
it is about 60 nt in length, has a very distinct predicted secondary structure and is expressed
very early and transiently after nitrogen step-down. Moreover, its expression requires HetR and
NtcA, and is restricted to cells that are differentiating into heterocysts, clearly placing NsiR1
within the regulon that controls the switch to nitrogen fixation and heterocyst formation. The
genomic arrangement of NsiR1, located upstream of hetF, a gene whose product is involved in
heterocyst formation, is conserved in all five Nostocales whose genomes are completely sequenced.
Additionally, we detected NsiR1 expression in 19 different heterocyst-forming cyanobacteria. Our
data suggest that every repeat is a complete transcriptional unit furnished with a cell-type
specific promoter and a Rho-independent terminator, that gives rise to a very high NsiR1 transcript
level. NsiR1 is the first known bacterial non-coding RNA that is specifically upregulated in
response to nitrogen step-down.post to:
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