The typographical corrections below were made to the article on the date noted. These changes have been incorporated in the article and the details are documented here.

9 December 1998:

In paragraph 6 of Results, "Figure 1" was changed to "Figure 2".

16 December 1998:

In paragraphs 7, 12, and 13 of Methods and in the legend of Figure 5, "chromatographed" was changed to "electrophoresed".