The puzzle of zmpB and extensive chain formation, autolysis defect and non-translocation of choline-binding proteins in Streptococcus pneumoniae

Choline-binding proteins (CBPs) from Streptococcus pneumoniae are involved in several important processes. Inactivation of zmpB, a gene that encodes a surface-located putative zinc metalloprotease, in a S. pneumoniae serotype 4 strain was recently reported to reveal a composite phenotype, including extensive chain formation, lysis defect and transformation deficiency. This phenotype was associated with the lack of surface expression of several CB Ps, including the major autolysin LytA. LytA, normally 36 kDa in size, was reported to form an SDS-resistant 80 kDa complex with CinA. ZmpB was theref ore proposed to control translocation of CBPs to the surface, possibly thro ugh the proteolytic release of CBPs (and RecA) from CinA. Based on the use of 12 independent mariner insertions in the zmpB gene of the well-character ized R6 laboratory strain, we could not confirm several of these observatio ns. Our zmpB mutants: (i) did not form chains; (ii) lysed normally in the p resence of deoxycholate, which indicates the presence of a functional autol ysin; (iii) transformed at normal frequency; and (iv) contained bona fide C inA and LytA species. Polymorphism of ZmpB between R6 and the serotype 4 is olate could not account for the discrepancy, as inactivation of zmpB (throu gh replacement by transposon-inactivated zmpB R6 alleles) in the latter str ain did not affect separation of daughter cells and autolysis. The conflict ing observations could be explained by our finding that the reportedly sero type 4 zmpB 'mutant' differed from its S. pneumoniae parent in lacking caps ule and in exhibiting characteristic traits of the Streptococcus viridans g roup, including resistance to optochin.