Amino acid and protein biosynthesis requires a number of high energy phosphate bonds and includes a dual energy cost for the synthesis of chemical intermediates during the fueling reactions and the conversion of precursor molecules to final products. One popular hypothesis is that the proteins encoded by putative highly expressed genes (hence called PHXPs) generally utilize low energy consuming amino acids to reduce the biosynthetic cost of the essential proteins. In our study, we found that this idea was not supported in the case of actinobacteria. With the actinobacteria, the energy costs of PHXPs varied in relation to their niche. Free-living, including aquatic, soil and extremophilic, and plant-associated actinobacteria were found to use energetically expensive amino acids in their PHXPs. An exception occurred with some animal-host-associated actinobacteria that used energy efficient amino acids. One explanation for these results may be due to the diverse metabolic patterns exhibited by actinobacteria under varied niches influenced by nutritional availability and physical environment.
