RECIPROCAL REGULATION OF THE DIFFERENTIATION OF MYXOCOCCUS-XANTHUS BYPKN5 AND PKN6, EUKARYOTIC-LIKE SER THR PROTEIN-KINASES/

Myxococcus xanthus contains a large family of genes encoding eukaryoti c-like serine/threonine kinases, Among them, two genes, pkn5 and pkn6, are divergently located on the chromosome and share a 46 bp promoter region between their transcription initiation sites, as determined by RNA protection, Pkn5, consisting of 380 amino acid residues, is a solu ble protein in the cytoplasm, while Pkn6, consisting of 710 amino acid residues, is a transmembrane protein, Its membrane topology was deter mined using the Pkn6-PhoA fusion protein in Escherichia coil, which ha s a single transmembrane domain with the N-terminal domain in the cyto plasm and the C-terminal domain outside the cytoplasmic membrane. Both proteins, when expressed in E. coli, were autophosphorylated: Pkn5 on ly at Ser, and PknG at both Ser and Thr. In M. xanthus, both genes are expressed constitutively throughout the life cycle, with slight incre ases at an early stage of development, Most strikingly, a pkn5-deletio n strain forms fruiting bodies much faster than the wild-type strain, while a pkn6-deletion strain develops slower than the wild-type strain , These results, together with the fact that the pkn5-deletion strain is able to form fruiting bodies on semi-rich media, suggest that Pkn5 and PknG have reciprocal roles in M. xanthus growth and development, F urthermore, PknG may be a transmembrane sensor of external signals for development, while Pkn5 is a kinase that negatively regulates M. xant hus development.