Genes in the accessory sec locus of Streptococcus gordonii have three functionally distinct effects on the expression of the platelet‐binding protein GspB

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Abstract
Platelet binding by Streptococcus gordonii strain M99 is strongly correlated with the expression of the large surface glycoprotein GspB. A 14 kb chromosomal region downstream of gspB was previously shown to be required for the expression of this protein. The region encodes SecA2 and SecY2, which are components of an accessory secretion system dedicated specifically to the export of GspB. The region also includes three genes (gly, nss and gtf) that encode proteins likely to function in carbohydrate metabolism, and four genes (orf1-4) that encode proteins of unknown function. In this report, we have investigated the role of these genes in GspB expression. We found that disruption of orf1, orf2 or orf3 resulted in a loss of GspB export and the intracellular accumulation of GspB. As they are apparently essential components of the accessory secretion system, these genes were renamed asp1-3 (for accessory secretory protein). In gtf and orf4 mutants, gspB was transcribed, but no GspB was detected. These results suggest that Gtf and Orf4 are required for the translation or for the stability of GspB. In contrast, gly and nss mutants were able to express and export GspB. However, disruption of these genes appeared to affect the carbohydrate composition of this glycoprotein. As asp1-3, gtf and orf4, but not gly and nss, are conserved in the accessory sec loci of several staphylococcal and streptococcal species, these genes may also have crucial roles in the expression and export of GspB homologues in the other Gram-positive bacteria.