The nucleotide sequence of the lacY gene coding for lactose permease (M protein) in Escherichia coli has been determined. The sequence includes the intergenic regions between the lacZ (beta-galactosidase) and lacY genes as well as the region between the lacY and lacA (transacetylase) genes. Lactose permease is predicted to consist of 417 residues (71% nonpolar), resulting in a protein with a molecular weight of 46,504. The reading frame was confirmed by the sequence of a nonsense mutation changing codon 33 from UGG to UAG.

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Mutations in the lacY gene of Escherichia coli have been used to analyze the functional organization of lactose permease. Deletions suggest that the NH2 terminus oflactose permease is not essential and can be replaced by residues of the cytoplasmic enzyme (3-galactosidase. Negative dominant mutations in the lacY gene can be explained by the assumption that membrane-associated lactose permease is active as a dimer or oligomer. The map positions of these mutations and other point mutations that lower or alter the sugar specificity define regions of lactose permease involved in sugar or proton binding and transport.

The deletion mutants b508 and b522 of bacteriophage lambda both end within the attachment site. The formation of such deletions is dependent upon the presence of intact integrase, and thus the deletion endpoints may be related to the normal crossover site in site-specific recombination. We have determined the DNA sequences of the attachment site regions of these deletions. Comparison of the sequences with lambda wildtype shows that both the deletions end within the central common homology region but at different positions. The consequences of these findings for current models of site-specific recombination are discussed.

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