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The Tra Domain of the Lactococcal CluA Surface Protein Is a Unique Domain That Contributes to Sex Factor DNA Transfer

Institute of Food Research, Norwich Research Park, Norwich NR4 7UA, United Kingdom

Received 23 September 2005/ Accepted 21 December 2005

CluA is a cell surface-presented protein that causes cell aggregation and is essential for a high-efficiency conjugation process in Lactococcus lactis. We know from previous work that in addition to promoting cell-to-cell contact, CluA is involved in sex factor DNA transfer. To define the CluA domains involved in aggregation and in transfer, we first performed random mutagenesis of the cluA gene using a modified mini-Tn7 element which generated five amino acid insertions located throughout the encoded protein. Thirty independent cluA insertion mutants expressing modified CluA proteins at the cell surface were isolated and characterized further. The level of aggregation of each mutant was determined. The cell binding capacity of CluA was affected strongly when the protein had a mutation in its N-terminal region, which defined an aggregation domain extending from amino acid 153 to amino acid 483. Of the cluA mutants that still exhibited aggregation, eight showed an attenuated ability to conjugate, and six mutations were located in a 300-amino-acid C-terminal region of the protein defining a transfer domain (Tra). This result was confirmed by a phenotypic analysis of an additional five mutants obtained using site-directed mutagenesis in which charged amino acids of the Tra domain were replaced by alanine residues. Two distinct functional domains of the CluA protein were defined in this work; the first domain is involved in cell binding specificity, and the Tra domain is probably involved in the formation of the DNA transport machinery. This is the first report of a protein involved in conjugation that actively contributes to DNA transfer and mediates contact between donor and recipient strains.