slpM, a gene coding for an "S-layer-like array" overexpressed in S-layer mutants of Thermus thermophilus HB8.

S-layer deletion mutants of Thermus thermophilus HB8 overproduce a regular array which surrounds groups of several cells. Averages of two-dimensional projections revealed a detailed architecture similar in general morphology and unit cell dimensions to that of the S-layer but having a different mass distribution. The structural components of these "S-layer-like arrays" are a group of three proteins of 52 (P52), 50 (P50), and 36 (P36) kDa, which are overexpressed in S-layer mutants. These three proteins specifically bind antibodies against P52, suggesting that the smaller proteins correspond to fragments derived from P52. This hypothesis was demonstrated by the identity of the trypsin digestion products of P52 and P50. The gene slpM, responsible for the synthesis of P52, was cloned by using synthetic oligonucleotides designed from partial amino acid sequences of P52 and P50. When slpM was expressed in Escherichia coli, proteins specifically recognized by anti-P52 antiserum whose electrophoretic mobilities were similar to those of P52 and P36 were detected. The sequence of slpM revealed the existence of an open reading frame in which the amino termini of P52, P50, and P36 were identified. The unprocessed product of slpM is a 469-amino-acid-long polypeptide whose theoretical M(r) (52,131) was in good agreement with the electrophoretic mobility of P52. The properties deduced for the product of slpM are very different from those of any S-layer protein so far sequenced. The possible roles of SlpM in wild-type cells are discussed.