SIGNAL-TRANSDUCTION AND TRANSCRIPTIONAL AND POSTTRANSCRIPTIONAL CONTROL OF IRON-REGULATED GENES IN BACTERIA

Iron is an essential element for nearly all living cells. Thus, the ab ility of bacteria to utilize ir on is a crucial survival mechanism ind ependent of the ecological niche in which the microorganism lives, bec ause iron is scarce both in potential biological hosts, where it is bo und by high-affinity iron-binding proteins, and in the environment, wh ere it is present as part of insoluble complex hydroxides. Therefore, pathogens attempting to establish an infection and environmental micro organisms must all be able to utilize the otherwise unavailable ir on. One of the strategies to perform this task is the possession of sider ophore-mediated iron uptake systems that are capable of scavenging the hoarded iron. This metal is, however, a double-edged sword for the ce ll because it can catalyze the production of deadly free hydroxyl radi cals, which are harmful to the cells. It is therefore imperative for t he cell to control the concentration of iron at levels that permit key metabolic steps to occur without becoming a messenger of cell death. Ea,;ly work identified a repressor, For, which as a complex with iron repressed the expression of most iron uptake systems as well as other iron-regulated genes when the iron concentration reached a certain lev el. However, later work demonstrated that this regulation by Fur was n ot the only answer: under low-iron conditions, there was a need for ac tivation of iron uptake genes as well as siderophore biosynthetic gene s. Furthermore, it was also realized that in some instances the actual ferric iron-siderophore complex induced the transcription of the cogn ate receptor and transport genes. It became evident that control of th e expression of iron-regulated genes was more complex than oiginally e nvisioned. In this review, I analyze the processes of signal transduct ion, transcriptional control, and posttranscriptional control of iron- regulated genes as, reported for the ferric dicitrate system in Escher ichia coli; pyochelin, pyoverdin, and enterobactin systems in Pseudomo nas species; the irgB system in Vibrio cholerae; and the plasmid-media ted anguibactin system in Vibrio anguillarum. I hope that by using the se diverse paradigms, I will be able to convey a unifying picture of t hese mechanisms and their importance in the maintenance and prosperity of bacteria within their ecological niches.