Global transcriptional analysis of Geobacter sulfurreducens under palladium reducing conditions reveals new key cytochromes involved

Abstract

Geobacter sulfurreducens is capable of reducing Pd(II) to Pd(0) using acetate as electron donor; however, the biochemical and genetic mechanisms involved in this process have not been described. In this work, we carried out transcriptome profiling analysis to identify the genes involved in Pd(II) reduction in this bacterium. Our results showed that 252 genes were upregulated while 141 were downregulated during Pd(II) reduction. Among the upregulated genes, 12 were related to energy metabolism and electron transport; 50 were classified as involved in protein synthesis; 42 were associated with regulatory functions and transcription, and 47 have no homologs with known function. RT-qPCR data confirmed upregulation of genes encoding PilA, the structural protein for the electrically conductive pili, as well as c-type cytochromes GSU1062, GSU2513, GSU2808, GSU2934, GSU3107, OmcH, OmcM, PpcA, PpcD under Pd(II)-reducing conditions. {Delta}pilA and {Delta}pilR mutant strains showed 20% and 40% decrease in the Pd(II)-reducing capacity, respectively, compared to the wild type strain, indicating the central role of the pili in this process. RT-qPCR data collected during Pd(II) reduction also confirmed downregulation of the omcB, omcC, omcZ, and omcS genes, which have been shown to be involved in Fe(III) reduction and electrodes. Based on these results, we propose mechanisms involved in Pd(II) reduction by G. sulfurreducens.\n\nImportanceGeobacter sulfurreducens is a versatile microorganism, known for its ability to reduce a wide range of environmentally relevant metals. It has been reported that this bacterium synthesizes palladium nanoparticles successfully. Yet, the biochemical and genetic mechanisms involved in this process had not been described previously. By using a transcriptome profile analysis to identify genes implicated in this process and genetic and physiological data, we propose a model for the biological reduction of Pd(II) by G. sulfurreducens. Moreover, the study also revealed the microbial reduction of Pd(II) coupled to growth, which shows not only unexpected ideas about its complex metabolism, but some key cytochromes involved in this pathway, which have not been previously associated with other metal reducing process for this bacterium. In brief the present work contributes to a better understanding of the biochemical and genetic mechanisms involved in Pd(II) reduction and put forward novel insights about G. sulfurreducens metabolism.