Title: Ecoparalogs and Pavlovian learning in environmental bacteria
Authors: Víctor de Lorenzo and A. David Páez.
Abstract:The composition and gene distribution in bacterial genomes often reflect the capacity of adaptation to different ecological niches. It is frequent to find a direct link between a genome and the environment where it operates. But sometimes there are important enigmas difficult to explain, including the presence/absence of certain genes, genetic redundancies and regulatory associations with no apparent logic. In this work we used Pseudomonas putida KT2440 to study some of these questions in a microorganism with some apparent genomic oddities in quest for a biological function. P. putida KT2440 is an ubiquitous saprophytic bacterium endowed with a remarkable adaptability to diverse environments. This bacterium bears two separate ars operons (encoding tolerance to arsenic) in the genome. The phenotypes determined by the duplicated ars1 and ars2 gene clusters of P. putida KT2440 (that endow the cell with hyper-resistance to arsenic) were exploited to understand the selective pressure behind the paradoxical and apparently stable coexistence of virtually identical copies of the same gene set. We could demonstrate that the contribution of each ars operon to the phenotype of As tolerance was not additive, as either cluster sufficed to endow cells with a high-level of resistance to the metalloid. However, otherwise identical traits linked to each of the ars sites diverged when temperature was decreased. Growth of the various mutants at 15°C (instead of the standard 30°C for P. putida) uncovered that ars2 afforded a much higher tolerance to As(V) and As(III) than the ars1 counterpart. Moreover, the examination of the genomic context of each of the ars operons also revealed that the presence of arsenic in the medium has been evolutionary recruited as a signal for regulating expression of genes unrelated to the toxicity of the oxyanion. This includes functions as different as tolerance to the herbicide phosphinothricin (PPT). On this basis, the capacity of P. putida KT2440 to endure exposure to metals/metalloids has to be understood from a wider evolutionary perspective, i.e. selection of its fine-tuning to face a toxic or its tinkering with other regulatory networks.
Questions & Comments: