A multidrug efflux response to methyl viologen and acriflavine toxicity in the cyanobacterium Synechocystis sp. PCC6803

Abstract

The genome of the model cyanobacterium Synechocystis sp. PCC6803 contains 28 genes encoding multidrug efflux transporters, including only two transporters from the multidrug and toxic compound extrusion (MATE) family (Slr0896 and Slr1543, NorM- and DinF-like, respectively) and one transporter of the bile acid-sodium symporter (BASS) type (Sll1428). In this study, single and double mutant Synechocystis sp. PCC6803 strains of slr0896, slr1543, and sll1428 were created. The mutant strains exhibited increased sensitivity to the toxic organic cations acriflavine and methyl viologen, but not fluoroquinolones, tetracycline, berberine, or sodium deoxycholate. To assess the involvement of multidrug transporters in the mitigation of acriflavine- and methyl viologen-induced toxicity, we performed transcript analysis of slr0896, slr1543, and sll1428, three ATP-binding cassette (ABC) (sll1180, slr1488, and slr1494), and three resistance-nodulation-cell division (RND) (slr0369, slr0454, and slr2131) transporter genes in wildtype and mutant Synechocystis sp. PCC6803 strains. Compared to the wildtype, under basal conditions the double mutants showed upregulation of the remaining broad-range Slr0896, Sll1428, or Slr1543 transporter/s; in addition, all mutants showed a very high upregulation of the ABC transporter Slr1488. Upregulation of slr0896, sll1428, and slr1543 in mutant strains exposed to acriflavine supported their role in its export. slr0896 is previously reported to be downregulated under methyl viologen exposure by the methyl viologen-responsive regulator gene slr0895, which is also evident in this study; in addition, we observed a downregulation of sll1428 and slr1543 in the presence of methyl viologen. The findings suggest that Slr1488 is an important transporter for maintenance of cellular homeostasis in response to exogenous toxic compounds, supported by the high levels of sll1488 expression present in strains deficient in multidrug resistance (MDR) transporters.