The Heterocyst-Specific NsiR1 Small RNA Is an Early Marker of Cell Differentiation in Cyanobacterial Filaments

Differentiation of single cells along filaments of cyanobacteria constitutes one of the simplest developmental patterns in nature. In response to nitrogen deficiency, certain cells located in a semiregular pattern along filaments differentiate into specialized nitrogen-fixing cells called heterocysts. The process involves the sequential activation of many genes whose expression takes place, either exclusively or at least more strongly, in those cells undergoing differentiation. In the model cyanobacterium Anabaena (Nostoc) sp. strain PCC 7120, increased transcription of hetR, considered the earliest detectable heterocyst-specific transcript, has been reported to occur in pairs or even in clusters of cells, thus making it difficult to identify prospective heterocysts during the early stages of differentiation, before any morphological change is detectable. The promoter of nsiR1 (nitrogen stress inducible RNA1), a heterocyst-specific small RNA, constitutes a minimal sequence promoting heterocyst-specific transcription. Using confocal fluorescence microscopy, I have analyzed expression of a gfp reporter transcriptionally fused to P(nsiR1). The combined analysis of green fluorescence (reporting transcriptional activity from P(nsiR1)) and red fluorescence (an indication of progress in the differentiation of individual cells) shows that expression of P(nsiR1) takes place in single cells located in a semiregular pattern before any other morphological or fluorescence signature of differentiation can be observed, thus providing an early marker for cells undergoing differentiation.