Effects of RyhB on Fur regulation of cytochrome c synthesis in Shewanella oneidensis

ABSTRACT Shewanella oneidensis is one of the most extensively studied electrochemically active bacteria because of its unparalleled respiratory versatility, which is largely built on diverse cytochromes (cyts) c . It has been firmly established that Fur, RyhB, and Hfq are critical for cyt c production through regulation of iron homeostasis, but how they interplay remains largely unknown. In this study, we show that S. oneidensis RyhB carries novel features, such as unusual length and functional exchangeability with its Escherichia coli counterpart but not its Vibrio cholerae counterpart. The RyhB loss suppresses the signature phenotypes caused by the Fur loss, including impaired growth and cyt c production in S. oneidensis . Despite this, neither the direct regulation of expression of most cyt c genes nor the suppression of disrupted iron homeostasis resulting from Fur loss appears to be a primary mechanism underlying RyhB’s influence on cyt c content. Rather, this effect is achieved by modulating heme levels; RyhB mediates the expression of the key genes in heme biosynthesis and the entire heme uptake/degradation system. Overall, our work highlights the significance of RyhB-mediated post-transcriptional regulation in the physiology of S. oneidensis , unraveling unexpected mechanisms by which RyhB affects cyt c biosynthesis. IMPORTANCE In bacteria, RyhB has been implicated in the regulation of diverse biological processes post-translationally. In Shewanella oneidensis , both Fur and RyhB influence the cytochrome (cyt) c content, which are the cornerstones of bacterial respiratory versatility and potential application in bioenergy and bioremediation. Here, we show that while RyhB generally exhibits an antagonistic relationship with Fur, it carries novel sequence features and differentially regulates heme biosynthesis and degradation, leading to altered cyt c content. The data presented show that the biological processes in a given bacterium regulated by RyhB are highly complex, amounting to the required coordination of multiple regulators to allow cells to respond to environmental changes promptly.