Two ATP phosphoribosyltransferase isozymes ofGeobacter sulfurreducenscontribute to growth in the presence or absence of histidine and under nitrogen fixation conditions

Abstract

Bacteria of the Geobacter clade possess two distinct ATP phosphoribosyltransferases encoded by hisGLand 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 hisGLand hisGS+hisZ encode functional ATP phosphoribosyltransferases. However, deletion of hisGLresulted in no growth defect, whereas deletion of hisGSdelayed growth when histidine was not provided. Both deletions increased hisZ transcript abundance, and both ΔhisGSand ΔhisZ mutations increased hisGLtranscript abundance. Growth with HisGLalone (due to deletion of either hisGSor 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, HisGLalso functions as ATP phosphoribosyltransferase, particularly during nitrogen fixation.