PopZ is dispensable for cell fitness but required for polar localization of ParB in Zymomonas mobilis.

Chromosome segregation is an essential process in monoploid bacteria, in which faithful inheritance of the genetic material is required for viability of daughter cell. In several alpha-proteobacteria, the polar organizing protein PopZ plays a crucial role in chromosome segregation by anchoring the chromosome partitioning machinery at a cell pole. While this process has been studied in monoploid species, it remains unclear whether this type of segregation mechanisms play a significant role in bacterial cells that carry multiple copies of chromosome. The alpha-proteobacterium Zymomonas mobilis exhibits ethanologenic physiology, and thus, is a promising chassis for biofuel production. Recent studies showed that Z. mobilis may serve as a non-model system for studying bacterial cell biology, owing to its relatively simple physiology and reduced genome size. Z. mobilis has previously been shown to be polyploid, and possesses a homolog of PopZ, a key regulator of chromosome organisation and cell polarity in alpha-proteobacteria. This raises the question of whether PopZ is involved in the organization of multiple chromosome copies. Here, we investigated the function of PopZ in Z. mobilis. Unlike in previously studied alpha-proteobacterial species, PopZ is dispensable for growth and cell morphology in Z. mobilis. However, the loss of PopZ altered the polar accumulation of chromosome segregation protein ParB, indicating an involvement in chromosome organization in Z. mobilis. These findings indicate that Z. mobilis might possess a chromosome segregation machinery that is dispensable under favourable growth conditions but might become important under certain conditions that reduce chomorome copy numbers.