Heterologous expression and self‐assembly of the S‐layer protein SbsA of Bacillus stearothermophilus in Escherichia coli

The cell surface of Bacillus stearothermophilus PV72 is covered by a regular surface layer (S‐layer) composed of a single species of protein, SbsA, with a molecular weight of 130 000. Recently, the sequence of the corresponding gene (sbsA) has been determined. The SbsA coding region including the signal sequence was cloned as a polymerase chain reaction (PCR) product into a low‐copy‐number vector under the transcriptional control of the λpL promoter. Expression of sbsA was shown to be thermally inducible from the resulting vector pBK4 in a strain of Escherichia coli expressing the λcI857 from the chromosome. As shown by ultrathin sectioning of whole cells and immunogold labelling using SbsA‐specific antibodies, expression of sbsA in E. coli led to accumulation of sheet‐like self‐assembling products of the protein in the cytoplasm. No SbsA protein was detected either in the periplasm or in the supernatant fractions. Long‐term expression of sbsA from pBK4, including in the late stationary phase, did not lead to degradation of SbsA.