Effects of PatU3 peptides on cell size and heterocyst frequency of Anabaena sp. PCC 7120.

patU, one of the genes specifically found in filamentous cyanobacteria, is required for the pattern formation in heterocyst-forming species. In Anabaena sp. PCC 7120, patU is split into patU5 and patU3, and only patU3 is involved in heterocyst patterning. Here, we report that PatU3 is also involved in control of cell size. A patU3 deletion mutant showed remarkably smaller cell size and much higher heterocyst frequency than that of the wild type. Yeast two-hybrid and pull-down assays demonstrated a direct interaction between PatU3 and the cell division protein Ftn6. Without the N-terminal 16-aa portion (MQERFQ AVIKRRLQ IH), PatU3 was no longer able to interact with Ftn6. This portion of PatU3 is also required for the interaction with PatN, a protein related to heterocyst differentiation/patterning. Addition of the 16-aa peptide or AVIKRRLQ-containing peptides restored the cell size and heterocyst frequency of a patU3 deletion mutant to normal or near wild-type level. PatU3(1-16aa)-GFP, the N-terminal 16-aa sequence fused with GFP, formed polar aggregates and peripheral patches in heterocysts of Anabaena 7120, whereas PatU3(1-198aa)-GFP showed a homogeneous distribution in the cytoplasm of all cells. The N-terminal AVIKRRL-containing sequence may function in intact PatU3, as a separate peptide, or both.IMPORTANCEPatU (or split into PatU5 and PatU3) is distributed in almost all filamentous cyanobacteria, including those that do not form heterocysts (except Pseudanabaena); however, its functions other than heterocyst differentiation/patterning have not been reported before. In this study, we found that PatU3 in Anabaena sp. PCC 7120 is involved in cell size determination. The N-terminal 16-aa sequence of PatU3 is required for control of cell size and interaction with the cell division protein Ftn6, and an octapeptide (aa7 ∼ aa14) within the 16-aa sequence can restore the cell size (and heterocyst frequency) of a patU3 deletion mutant to normal. Such a peptide, if generated from PatU or PatU3 in vivo, may promote intercellular coordination in filamentous cyanobacteria.