Characterization of ssfR and ssgA, two genes involved in sporulation of Streptomyces griseus.

In the presence of cefoxitin, which inhibits septum formation during sporulation, Streptomyces griseus is unable to sporulate, retaining the sonication sensitivity of nonsporulating hyphae. Cefoxitin- and sonication-resistant mutant SKK2600 was isolated and showed many morphological differences from its parental strain. A 3.6-kb DNA fragment that complemented the mutations of SKK2600 contained two open reading frames (ORFs), either of which could complement SKK2600. One ORF, designated ssfR, encoded a protein containing a potential DNA-binding helix-turn-helix motif close to its N terminus. SsfR is similar to members of a large family of transcriptional regulators, particularly IclR of Escherichia coli. The second ORF was identified as ssgA, a previously described sporulation gene from S. griseus (S. Kawamoto and J. C. Ensign, Actinomycetology 9:136-151, 1995). A point mutation of C to T seven nucleotides upstream of the UGA stop codon of ssfR was responsible for the phenotype of isolated mutant strain SKK2600. Surprisingly, this mutation should not change the primary structure of SsfR. The ssfR and ssgA disruption mutants were constructed and showed the "white" mutant phenotype, with some growth medium dependence. In addition, the ssfR null mutant sporulated ectopically in phosphate starvation medium.