Construction and use of signal sequence selection vectors in Escherichia coli and Bacillus subtilis

To study the diversity and efficiency of signal peptides for secreted proteins in gram-positive bacteria, two plasmid vectors were constructed which were used to probe for export signal-coding regions in Bacillus subtilis. The vectors contained genes coding for extracellular proteins (the alpha-amylase gene from Bacillus licheniformis and the beta-lactamase gene from Escherichia coli) which lacked a functional signal sequence. By shotgun cloning of restriction fragments from B. subtilis chromosomal DNA, a great variety of different export-coding regions were selected. These regions were functional both in B. subtilis and in E. coli. In a number of cases where protein export had been restored, intracellular precursor proteins of increased size could be detected, which upon translocation across the cellular membrane were processed to mature products. The high frequency with which export signal-coding regions were obtained suggests that, in addition to natural signal sequences, many randomly cloned sequences can function as export signal.