CLONING OF THE RHODOBACTER-SPHAEROIDES HISI GENE - UNIFUNCTIONALITY OF THE ENCODED PROTEIN AND LACK OF LINKAGE TO OTHER HIS GENES

The Rhodobacter sphaeroides 2.4.1 hisl gene, which encodes a phosphori bosyl-AMP-cyclohydrolase that catalyses the third step in the histidin e biosynthetic pathway, has been isolated from a genomic library of th is phototrophic bacterium by complementation of an Escherichia coil hi sl mutant. Analysis of the nucleotide sequence of the R. sphaeroides h isl gene reveals that it encodes a deduced product of 119 aa with a pr edicted molecular mass of 13.4 kDa. In contrast to the situation in E. coil, the R. sphaeroides hisl gene encodes a unifunctional protein an d it is not linked to the hisE gene. The absence of a single histidine operon like that of E. coil was confirmed by PFGE experiments and com plementation analysis of a R. sphaeroides hisl mutant that was constru cted by marker exchange. The location of hisl in the R. sphaeroides ge nome has been determined to be at map co-ordinate 2275+/-20 of chromos ome I.