Genetic organization of the streptokinase region of the Streptococcus equisimilis H46A chromosome

Abstract

The complete nucleotide sequences of four genes and one open reading frame (ORF1) adjacent to the streptokinase gene, skc, from Streptococcus equisimilis H46A were determined. These genes are encoded on the opposite DNA strand to skc and are arranged as follows: dexB-abc-lrp-skc-ORF1-rel. The dexB gene, coding for an alpha-glucosidase (M(r) 61,733), and abc, encoding an ABC transporter (M(r) 42,080), are similar to the dexB and msmK genes, respectively, from the multiple sugar metabolism operon of S. mutans. The lrp gene specifies a leucine-rich protein (M(r) 32,302) that has a leucine-zipper motif at its C-terminus. The function of the Lrp protein is not known but appeared to be detrimental when overexpressed in Escherichia coli. Although lrp appears not to be an essential gene, as judged by plasmid insertion mutagenesis, it is conserved in all streptococcal strains carrying a streptokinase gene. The rel gene showed significant homology to the E. coli relA and spoT genes involved in the stringent response to amino acid deprivation. Multiple alignment of the amino acid sequences of Rel (M(r) 83,913), RelA and SpoT revealed 59.4% homology of the primary structures. Northern hybridization analyses of the genes in the skc region showed skc to be transcribed most abundantly. In addition to transcripts for skc, monocistronic mRNAs were detected for all three genes divergently transcribed from skc. Although there was also some read-through transcription from lrp into abc, and from abc into dexB, the transcription pattern suggests a high degree of transcriptional and functional independence not only of skc but also abc and dexB. Prominent structural features in intergenic regions included a static DNA bending locus located upstream and a putative bidirectional transcription terminator downstream of skc.