THE P-II PROTEIN IN THE CYANOBACTERIUM SYNECHOCOCCUS SP STRAIN PCC-7942 IS MODIFIED BY SERINE PHOSPHORYLATION AND SIGNALS THE CELLULAR N-STATUS

The glnB gene product (P-II protein) from synechococcus sp. has previo usly been identified among P-32-labeled proteins, and its modification state has been observed to depend on both the nitrogen source and the spectral light quality (N.F. Tsinoremas, A.M. Castets, M.A. Harrison, J.F. Allen, and N. Tandeau de Marsac, Proc. Natl. Acad. Sci. USA 88:4 565-4569, 1991). As shown in this study, modification of the P-II prot ein primarily responds to the N-status of the cell, and its light-depe ndent variations are mediated through nitrate metabolism. Modification of the P-II protein results in the appearance of three isomeric forms with increasing negative charge. Unlike its homolog counterparts char acterized so far, P-II in synechococcus sp. is modified by phosphoryla tion on a serine residue, which represents a unique kind of protein mo dification in bacterial nitrogen signalling pathways.