Identification of phosphoproteins in Escherichia coli

The substrates of ion- and lipid-stimulated protein kinase activity in extracts of Escherichia coli were purified by chromatography. Subsequent N-terminal sequencing suggests that these substrates include the following: a novel 80 kDa protein co-purifying with RNA polymerase but partially homologous to elongation factor G; a protein with an apparent molecular weight of 65 kDa identified as the ribosomal protein S1; and a 32 kDa protein identified as succinyl CoA synthetase, a key enzyme in the tricarboxylic acid cycle. The phosphorylation of these three proteins was markedly stimulated by the addition of manganese, and occurred on threonine, serine or tyrosine residues as indicated by the stability of the phosphoresidues during acid treatment. In addition, a calcium-stimulated protein of 70 kDa was identified as the heat-shock protein DnaK, and a 17 kDa lipid-stimulated phosphoprotein as nucleotide diphosphate kinase.