Characterisation of two putative protein Ser/Thr kinases from actinomycete Streptomyces granaticolor both endowed with different properties

The structural genes, pkg4 and pkg3, encoding two putative protein serine/threonine kinases in Streptomyces granaticolor; have been cloned and sequenced. The genes were isolated after screening genomic sublibraries with specific probes obtained by PCR amplification of chromosomal DNA using: degenerate primers which correspond to amino acid sequences highly conserved in eukaryotic protein Ser/Thr kinases. The sequences of these genes predict polypeptide chains of 761 and 780 amino acids for Pkg4 and Pkg3, respectively. The genes are separated by only 2 bp and therefore probably constitute an operon. pkg4, which is positioned upstream of pkg3, contains a UUA(Leu) codon suggesting a developmental-dependent mode of expression. The amino-terminal half of both proteins clearly shares similarities with the family of protein Ser/Thr kinases. Both proteins studied also possess a region rich in Pro and Ala residues and a repeating motif of 11 amino acid residues, the function of which is unknown, in the carboxy-terminal domain. Expression of pkg4 in Escherichia coli gave rise to two different forms: a soluble protein autophosphorylated at threonine residues and an insoluble form phosphorylated at threonine and serine residues. In contrast, when pkg3 was expressed in E. coli, no autophosphorylation was detected either in vivo or in vitro.