The mannose permease of the bacterial phosphotransferase system mediates sugar transport across the cytoplasmic membrane concomitant with sugar phosphorylation. It also functions as a receptor for bacterial chemotaxis and is required for infection of the cell by bacteriophage lambda where it most likely functions as a pore for penetration of lambda DNA. The permease consists of three different subunits, IIIMan, II-PMan, and II-MMan, which are encoded in a single transcriptional unit ptsLPM. The complete amino acid sequence of the subunits is deduced from the nucleotide sequence. IIIMan (35 kDa) is a hydrophilic protein which is transiently phosphorylated and most likely contains the active site for sugar phosphorylation. II-PMan (28 kDa) is very hydrophobic; II-MMan (31 kDa) is moderately hydrophobic. Both are integral membrane proteins and most likely form the transmembrane channel. All three subunits are required for sugar transport and phosphorylation; II-PMan and II-MMan alone are sufficient for penetration of lambda DNA. Truncated forms of II-MMan and II-PMan are described that mediate lambda DNA penetration but have no apparent sugar transport activity. Residual sugar phosphorylation activity is found with the truncated form of II-PMan. No obvious homologies at the level of amino acid sequence could be detected with other bacterial transport proteins.