The robustness of porin-cytochrome gene clusters from in extracellular electron transfer.

UNLABELLED

To investigate their roles in extracellular electron transfer (EET), the porin-cytochrome () gene clusters , , and of the Gram-negative bacterium were deleted. Failure to delete all gene clusters at the same time suggested their essential roles in extracellular reduction of Fe(III)-citrate by . Deletion of had no impact on bacterial reduction of Fe(III)-citrate but diminished bacterial reduction of ferrihydrite and abolished anode reduction and direct interspecies electron transfer (DIET) to and . Although it had no impact on the bacterial reduction of Fe(III)-citrate, deletion of delayed ferrihydrite reduction, abolished anode reduction, and diminished DIET. Deletion of had little impact on DIET but diminished bacterial reductions of Fe(III)-citrate, ferrihydrite, and anodes. Most importantly, deletions of both and restored bacterial reduction of ferrihydrite and anodes and DIET. Enhanced expression of in this double mutant when grown in coculture with ΔΔ suggested that this cluster might compensate for impaired EET functions of deleting and . Thus, these gene clusters played essential, distinct, overlapping, and compensatory roles in EET of that are difficult to characterize as deletion of some clusters affected expression of others. The robustness of these gene clusters enabled to mediate EET to different acceptors for anaerobic growth even when two of its three gene clusters were inactivated by mutation. The results from this investigation provide new insights into the roles of gene clusters in bacterial EET.

IMPORTANCE

The Gram-negative bacterium is of environmental and biotechnological significance. Crucial to the unique physiology of is its extracellular electron transfer (EET) capability. This investigation sheds new light on the robust roles of the three porin-cytochrome () gene clusters, which are directly involved in EET across the bacterial outer membrane, in the EET of . In addition to their essential roles, these gene clusters also play distinct, overlapping, and compensatory roles in the EET of . The distinct roles of the gene clusters enable to mediate EET to a diverse group of electron acceptors for anaerobic respirations. The overlapping and compensatory roles of the gene clusters enable to maintain and restore its EET capability for anaerobic growth when one or two of its three gene clusters are deleted from the genome.