Upstream short sequence repeats regulate expression of the alpha C protein of group B Streptococcus

Abstract

Group B streptococci (GBS) express a family of repeat-containing surface proteins, the prototype of which is the alpha C protein expressed in type Ia/C strain A909. We have isolated a series of mutant GBS strains by mouse-passage of A909 that do not produce normal levels of the alpha C protein. Polymerase chain reaction amplification and sequencing of the gene encoding the alpha C protein, bca, from four mutant strains revealed the presence of a full-length gene in each strain. However, Northern and RT-PCR analysis revealed greatly reduced levels of RNA encoding the alpha C protein. Sequence analysis of the mutant genes found the coding region unchanged from the wild-type gene in each case, but variation was observed in a specific locus located 110 bp upstream of the start codon. The presence of a 5-nucleotide repeat, AGATT, and a string of adenine residues mark this locus. Both deletion and expansion of the AGATT motif were associated with the complete null phenotype. Deletions in the string of adenine residues were associated with both a decreased-production phenotype and a complete null phenotype. Cloning of this upstream region into a green-fluorescent protein (GFP) reporter system in GBS demonstrated promoter activity that was completely abolished by changes in the pentanucleotide repeat or adenine string. Primer extension studies of the wild-type strain revealed one dominant and two minor transcription start sites. Primer extension studies of the null and low-expression mutant strains revealed that the dominant transcript is completely absent in each mutant. The short sequence repeat locus is located at position -55 to -78 relative to the start site of the dominant transcript. We have demonstrated in vitro phase variation in expression of the alpha C protein associated with variation at the pentanucleotide repeat locus. We conclude that this short sequence repeat motif is located upstream of the dominant promoter for the alpha C protein and represents a regulatory site for alpha C protein expression. This is the first evidence of transcriptional regulation by short-sequence repeats in a Gram-positive organism.