The ltp gene of temperate Streptococcus thermophilus phage TP-J34 confers superinfection exclusion to Streptococcus thermophilus and Lactococcus lactis

The Itp gene, located within the lysogeny module of temperate Streptococcus thermophilus phage TP-J34, has been shown to be expressed in lysogenic strain S. thermophilus J34. It codes for a lipoprotein, as demonstrated by inhibition of cleavage of the signal sequence by globomycin. Exposure of Lip on the surface of Lactococcus lactis protoplasts bearing a plasmid-encoded copy of ltp has been demonstrated by immunogold labeling and electron microscopy. Expression of Itp in prophage- and plasmid-cured S. thermophilus J34-6f interfered with TP-J34 infection. While plating efficiency was reduced by a factor of about 40 and lysis of strain J34-6f in liquid medium was delayed considerably, phage adsorption was not affected at all. Intracellular accumulation of phage DNA was shown to be inhibited by Ltp. This indicates interference of Lip with infection at the stage of triggering DNA release and injection into the cell, indicating a role of Up in superinfection exclusion. Expression of Itp in L. lactis Bu2-60 showed that the same superinfection exclusion mechanism was strongly effective against phage P008, a member of the lactococcal 936 phage species: no plaque-formation was detectable with even 109 phage per ml applied, and lysis in liquid medium did not occur. In Lactococcus also, Lip apparently inhibited phage DNA release and/or injection. Lip appears to be a member of a family of small, secreted proteins with a 42 amino acids repeat structure encoded by genes of Gram-positive bacteria. Some of these homologous genes are part of the genomes of prophages. (c) 2006 Elsevier Inc. All rights reserved.