Cyanobacteria are prokaryotic organisms that carry out oxygenic photosynthesis. The fresh water cyanobacterium Synechococcus elongatus PCC 7942 is a model organism for the study of photosynthesis and gene regulation, and for biotechnological applications. Besides several freshwater cyanobacteria, S. elongatus 7942 also contains multiple chromosomal copies per cell at all stages of its cell cycle. Here, we describe a method for the direct visualization of multicopy chromosomes in S. elongatus 7942 by fluorescence in situ hybridization (FISH).

Cyanobacteria are prokaryotic organisms that perform oxygenic photosynthesis. Freshwater cyanobacteria, such as Synechococcus elongatus PCC 7942 and Synechocystis sp. PCC 6803, are model organisms for the study of photosynthesis, gene regulation, and biotechnological applications because they are easy to manipulate genetically. However, while studying these cyanobacteria, care has to be taken with respect to genetic heterogeneity in the establishment of gene disruptants, because these cyanobacteria contain multiple chromosomal copies per cell. Here, we describe a method for the estimation of chromosomal copy number in Synechococcus 7942. Using this method, we have recently observed that the chromosomal copy number of Synechococcus 7942 significantly changes during its growth phases. This technique is available for studying polyploidy not only in cyanobacteria, but also in other polyploid organisms.