To determine the effects of the extra N-terminal seven-amino-acid sequence on the function of chitosanase CsnA. Sequence and structure analysis indicated that the mature CsnA contains a seven-amino-acid extension in a disordered form at the N-terminus. To determine the function of this sequence, both mature CsnA and its N-terminus-truncated mutant, CsnA Delta N, were expressed in Escherichia coli and characterized. Compared with CsnA Delta N, CsnA exhibited a 15 A degrees C higher temperature optimum, enhanced pH stability, thermostability and catalytic efficiency. The underlying mechanisms responsible for these changes were analyzed by circular dichroism (CD) spectroscopy. CD analysis revealed that the deletion of the N-terminal sequence resulted in a decrease in the T-m of 4.3 A degrees C and this sequence altered the secondary structure of the enzyme. The N-terminal sequence is essential for the stability and activity of chitosanase CsnA.