Two ATP phosphoribosyltransferase isozymes of Geobacter sulfurreducens contribute to growth in the presence or absence of histidine and under nitrogen fixation conditions
Bacteria of the Geobacter clade possess two distinct ATP phosphoribosyltransferases encoded by hisGL and hisGS+hisZ to catalyze the first reaction of histidine biosynthesis. This very unusual redundancy was investigated by mutational analysis. The hisGL, hisGS, and hisZ genes of Geobacter sulfurreducens were deleted, effects on growth and histidine biosynthesis gene expression were evaluated, and deficiencies were complemented with plasmid-borne genes. Both hisGL and hisGS+hisZ encode functional ATP phosphoribosyltransferases. However, deletion of hisGL resulted in no growth defect, whereas deletion of hisGS delayed growth when histidine was not provided. Both deletions increased hisZ transcript abundance, and both ΔhisGS and ΔhisZ mutations increased hisGL transcript abundance. Growth with HisGL alone (due to deletion of either hisGS or hisZ) was better under nitrogen fixation conditions than when ammonium was provided. Deletion of hisZ caused growth defects under all conditions tested, with or without exogenous sources of histidine, with different patterns of histidine biosynthesis gene expression under each condition. Taken together, the data indicate that G. sulfurreducens depends primarily on the HisGSZ isozyme as an ATP phosphoribosyltransferase in histidine biosynthesis, and for other functions when histidine is available; however, HisGL also functions as ATP phosphoribosyltransferase, particularly during nitrogen fixation.