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

Abstract

ABSTRACT To investigate their roles in extracellular electron transfer (EET), the porin-cytochrome ( pcc ) gene clusters Gmet0825-0828 , Gmet0908-0910 , and Gmet0911-0913 of the Gram-negative bacterium Geobacter metallireducens were deleted. Failure to delete all pcc gene clusters at the same time suggested their essential roles in extracellular reduction of Fe(III)-citrate by G. metallireducens . Deletion of Gmet0825-0828 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 Methanosarcina barkeri and Geobacter sulfurreducens . Although it had no impact on the bacterial reduction of Fe(III)-citrate, deletion of Gmet0908-0910 delayed ferrihydrite reduction, abolished anode reduction, and diminished DIET. Deletion of Gmet0911-0913 had little impact on DIET but diminished bacterial reductions of Fe(III)-citrate, ferrihydrite, and anodes. Most importantly, deletions of both Gmet0825-0828 and Gmet0908-0910 restored bacterial reduction of ferrihydrite and anodes and DIET. Enhanced expression of Gmet0911-0913 in this double mutant when grown in coculture with G. sulfurreducens Δ hybL Δ fdnG suggested that this cluster might compensate for impaired EET functions of deleting Gmet0825-0828 and Gmet0908-0910 . Thus, these pcc gene clusters played essential, distinct, overlapping, and compensatory roles in EET of G. metallireducens that are difficult to characterize as deletion of some clusters affected expression of others. The robustness of these pcc gene clusters enabled G. metallireducens to mediate EET to different acceptors for anaerobic growth even when two of its three pcc gene clusters were inactivated by mutation. The results from this investigation provide new insights into the roles of pcc gene clusters in bacterial EET. IMPORTANCE The Gram-negative bacterium Geobacter metallireducens is of environmental and biotechnological significance. Crucial to the unique physiology of G. metallireducens is its extracellular electron transfer (EET) capability. This investigation sheds new light on the robust roles of the three porin-cytochrome ( pcc ) gene clusters, which are directly involved in EET across the bacterial outer membrane, in the EET of G. metallireducens . In addition to their essential roles, these gene clusters also play distinct, overlapping, and compensatory roles in the EET of G. metallireducens . The distinct roles of the pcc gene clusters enable G. metallireducens to mediate EET to a diverse group of electron acceptors for anaerobic respirations. The overlapping and compensatory roles of the pcc gene clusters enable G. metallireducens to maintain and restore its EET capability for anaerobic growth when one or two of its three pcc gene clusters are deleted from the genome.