Effects of PatU3 Peptides on Cell Size and Heterocyst Frequency of sp. Strain PCC 7120.

, one of the genes specifically found in filamentous cyanobacteria, is required for the pattern formation in heterocyst-forming species. In sp. strain PCC 7120, is split into and , and only is involved in heterocyst patterning. Here, we report that PatU3 is also involved in control of cell size. A deletion mutant showed remarkably smaller cell size and much higher heterocyst frequency than the wild type. Yeast two-hybrid and pulldown assays demonstrated a direct interaction between PatU3 and the cell division protein Ftn6. Without the N-terminal 16-amino-acid (aa) portion (MQERFQIH [the identified octapeptide is underlined]), 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 deletion mutant to normal or nearly wild-type levels. PatU3(1-16aa)-GFP, the N-terminal 16-aa sequence fused with green fluorescent protein (GFP), formed polar aggregates and peripheral patches in heterocysts of sp. strain PCC 7120, whereas PatU3(1-198aa)-GFP showed a homogeneous distribution in the cytoplasm of all cells. The N-terminal AVIKRRLQ-containing sequence may function in intact PatU3, as a separate peptide, or both. PatU (or split into PatU5 and PatU3) is distributed in almost all filamentous cyanobacteria, including those that do not form heterocysts (except ); however, its functions other than heterocyst differentiation/patterning have not been reported before. In this study, we found that PatU3 in sp. strain PCC 7120 is involved in cell size determination. The N-terminal 16-aa sequence of PatU3 is required for the control of cell size and interaction with the cell division protein Ftn6, and an octapeptide (aa 7 to aa 14) within the 16-aa sequence can restore the cell size (and heterocyst frequency) of a deletion mutant to normal. Such a peptide, if generated from PatU or PatU3 , may promote intercellular coordination in filamentous cyanobacteria.