Abstract
Small regulatory RNAs (sRNAs) are post-transcriptional regulators of bacterial gene expression. In cyanobacteria, the responses to nitrogen availability, that are mostly controlled at the transcriptional level by NtcA, involve also at least two small RNAs, namely NsiR4 (nitrogen stress-induced RNA 4) and NsrR1 (nitrogen stress-repressed RNA 1). Prediction of possible mRNA targets regulated by NsrR1 in Nostoc sp. PCC 7120 allowed, in addition to previously described nblA, the identification of all1871, a nitrogen-regulated gene encoding a protein of unknown function that we describe here as required for growth at the expense of atmospheric nitrogen (N2). We show that transcription of all1871 is induced upon nitrogen step-down independently of NtcA. All1871 accumulation is repressed by NsrR1 and its expression is stronger in heterocysts, specialized cells devoted to N2 fixation. We demonstrate specific interaction between NsrR1 and the 5′ untranslated region (UTR) of the all1871 mRNA, that leads to decreased expression of all1871. Because transcription of NsrR1 is partially repressed by NtcA, post-transcriptional regulation by NsrR1 would constitute an indirect way of NtcA-mediated regulation of all1871.
Highlights
Small non-coding RNAs are relevant players in regulatory circuits affecting essentially every aspect of bacterial physiology
To verify the interaction between NsrR1 and the mRNA of all1871, we used a heterologous reporter assay in E. coli [18], in which the 50 -untranslated region (UTR) of all1871, from the transcriptional start site (TSS) at position −137 with respect to the initiation codon (Mitschke et al, 2011), plus 60 nucleotides of the coding sequence of all1871 were fused to the sfgfp gene and co-expressed in E. coli with NsrR1 or with a control RNA
In E. coli, members of the CRP/FNR family of transcriptional regulators are known to control the expression of several small RNAs, all of them contributing to the regulatory effects exerted by these two major transcription factors [2]
Summary
Small non-coding RNAs (sRNAs) are relevant players in regulatory circuits affecting essentially every aspect of bacterial physiology. These types of molecules are usually post-transcriptional regulators fine-tuning the responses to different environmental conditions [1]. NtcA has been shown to regulate expression of several sRNAs, some of them with a wide distribution among phylogenetically distant cyanobacteria [5,8,9]. Among these sRNAs, NsiR4 (nitrogen-stress inducible RNA 4) is involved in nitrogen assimilation control via regulation of IF7, the inactivating factor of the key enzyme glutamine synthetase [8]. NsrR1 (nitrogen-stress repressed RNA 1) modulates translation of NblA [10], a protein
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