OUTER-MEMBRANE LOCALIZATION OF MUREIN HYDROLASES - MLTA, A 3RD LIPOPROTEIN LYTIC TRANSGLYCOSYLASE IN ESCHERICHIA-COLI

Lytic transglycosylases are a unique lysozyme-like class of murein hyd rolases believed to be important for growth of Escherichia coli. A mem brane-bound lytic transglycosylase with an apparent molecular mass of 38 kDa, which was designated Mlt38, has previously been purified and c haracterized (A. Ursinus and J.-V. Holtje, J. Bacteriol. 176:338-343, 1994). On the basis of four tryptic peptides, the gene mltA was mapped at 63 min on the chromosomal map of E. coli K-12 and cloned by revers e genetics. The open reading frame was found to contain a typical lipo protein consensus sequence, and the lipoprotein nature of the gene pro duct was demonstrated by [H-3] palmitate labelling. On the basis of th e distribution of MltA in membrane fractions obtained by sucrose gradi ent centrifugation, a localization in the outer membrane is indicated, Overexpression of MltA at 30 degrees C, the optimal temperature for e nzyme activity, but not at 37 degrees C results in the formation of sp heroplasts. Not only a deletion mutant in mltA, but also double mutant s in mltA and one of the two other well-characterized lytic transglyco sylases (either sltY or mltB), as well as a triple mutant in all three enzymes, showed no obvious phenotype. However, dramatic changes in th e structure of the murein sacculus indicate that lytic transglycosylas es are involved in maturation of the murein sacculus.