Identification, cloning, and nucleotide sequence of a silent S-layer protein gene of Lactobacillus acidophilus ATCC 4356 which has extensive similarity with the S-layer protein gene of this species

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Abstract
The bacterial S-layer forms a regular structure, composed of a monolayer of one (glyco)protein, on the surfaces of many prokaryotic species. S-layers are reported to fulfil different functions, such as attachment structures for extracellular enzymes and major virulence determinants for pathogenic species. Lactobacillus acidophilus ATCC 4356, which originates from the human pharynx, possesses such an S-layer. No function has yet been assigned to the S-layer of this species. Besides the structural gene (slpA) for the S-layer protein (S-protein) which constitutes this S-layer, we have identified a silent gene (slpB), which is almost identical to slpA in two regions. From the deduced amino acid sequence, it appears that the mature SB-protein (44,884 Da) is 53% similar to the SA-protein (43,636 Da) in the N-terminal and middle parts of the proteins. The C-terminal parts of the two proteins are identical except for one amino acid residue. The physical properties of the deduced S-proteins are virtually the same. Northern (RNA) blot analysis shows that only the slpA gene is expressed in wild-type cells, in line with the results from sequencing and primer extension analyses, which reveal that only the slpA gene harbors a promoter, which is located immediately upstream of the region where the two genes are identical. The occurrence of in vivo chromosomal recombination between the two S-protein-encoding genes will be described elsewhere.