Research

The genetics of histidine utilization

I would never have thought that the genetics of food acquisition could be interesting, but daily it becomes more fascinating. Of course, with hindsight, it is obvious that it should be interesting: nothing is more important than getting food.

Xue-Xian Zhang and PBR began this work some years ago as an antidote to years of trying to do genetics with minimal phenotypes. Put another way, the histidine uptake and utilization (hut) operon of SBW25 is active in the plant environment and inactive in minimal medium in the laboratory, but it can be activated simply by the addition of histidine (or its immediate breakdown product, urocanate).

Actually there was another reason for getting interested in this locus: an examination of the locus from different genome-sequenced Pseudomonas strains showed much polymorphism in the predicted transporters. The hut locus of SBW25 is particular complex with a histidine permease, a urocanate permease, an ABC system and various other transporters elsewhere in the genome -- some of which are tied to to response regulators. The causes of the differences among strains (in the uptake systems) suggested to us that this was where the action lay: that selection might work primarily on the uptake system. The population genetics of this is now something that we are heavily involved in.

Equally complex is the regulation of hut. It contains at least two levels of regulation: one specific and one general. At the specific level expression is controlled by the HutC repressor whose activity is modulated by urocanate (when urocanate is available repression is relieved). Also operating at the specific level is a newly identified regulator (HutD) that we suggest is a governor of hut that prevents the locus exceeding a critical upper level of activity. The need for the governor (we suggest) stems from the liberation of two molecules of ammonia for each molecule of histidine: too much ammonia is not good. And the postiive feed forward regulation of hut means that expression could run unchecked (and to dangerous levels) if there was no means of controlling the upper level of expression (see Zhang & Rainey (2007) Genetics 176, 2165). The possibility that HutD might be a therapeutically useful target is currently being investigated by Yunhao Liu.

At the level of general regulation hut is controlled by CbrAB (see Jonathan Gauntlett): CbrB is a sigma 54 enhancer binding protein that can activate hut transcription in response to both carbon and nitrogen starvation conditions. The hut locus is also regulated by the NtrBC two-component regulatorry system that ensures expression when cells are nitrogen starved (see Zhang & Rainey (2008) 178, 185). The ability of CbrAB to regulate expression of hut across a wide range of C:N ensures that histidine can be utilized whatever the carbon to nitrogen ratio. In essence, we show that SBW25 is a glutton; that its regulation is geared toward gaining maximum food as quickly as possible: "an eat now, pay later and sod the neighbour" approach to life.

Our recent work focusses largely on CbrA -- and takes place in collaboration with Greg Cook -- but also tackles the evolutionary causes of regulatory complexity. We're also beginning to recognize the challenges that organisms face when they are presented with a food source (such as histidine) that delivers both C and N; and when the organism has to deal with extracting C under conditions when it doesn't require N, and vice versa. It is fascinating stuff and motivates some interesting selection experiments.

And then there is the mystery of urocanate: where does it come from (apart from dog urine)?

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