Effect of protease mutations on the production of xylanases in Streptomyces lividans

Three protease mutants - 7 (tap(-)), 12 (tap(-), ssp(-)), and 17 (multiple mutations) - of Streptomyces lividans were tested for their influence on protein secretion. Streptomyces lividans grown in xylan secretes 3 xylanases (A, B, and C). Xylanases A (XlnA) and B (XlnB) are secreted by the See pathway, whereas xylanase C (XInC) is secreted by the Tat pathway. The production of XlnA and XlnC was affected in the mutants, suggesting that the mutations interfered with both See- and Tat-secretion systems. However, the processing rate for the See and Tat precursor was similar to the wildtype strain, indicating that the mutations had no direct effect on secretion. Streptomyces lividans naturally produced 2 forms of XlnB: XlnB1, which contains the catalytic and the xylan-binding domains, and XlnB2, which contains the catalytic domain only. There was no change from the wild-type strain in the ratio of XlnB1/XlnB2 produced by the mutants, indicating that these proteases are not involved in this process. Although XlnA1, partially truncated in its xylan-binding domain, was rapidly degraded to its catalytic domain (XlnA2) in the wild-type strain, the rate of conversion was reduced in the 3 mutants, indicating that the proteases participated to some extent in this proteolytic process.