SF213-224 were constructed as follows. First the entire BglII-NcoI fragment of pSF2 was replaced by a synthetic dsDNA fragment (5 0 -GATCTTAATACGACTCAC TATA-GGGCTCATTTAAACTAGTTGAGGGGCCA TATTTCGC-3 0 and 5 0 -CATGGCGAAATATGGCCC CTCAACTAGTTTAAATGAGCCCTATAGTGAGTC GTATTAA-3 0 , sequences forming a SpeI restriction site are underlined). The resulting plasmid pSF208 was opened at the BglII and SpeI sites and oligonucleotides SF226 (5 0 -GATCTTAATACGACTCACT-ATAGGGCTCAGG GAAA-3 0 ) and SF227 (5 0 -CTAGTTTCCCTGAGCCCT ATA-GTGAGTCGTATTAA-3 0 ) were inserted. This yielded pSF212, which was subsequently digested with SpeI and NcoI to insert sets of complementary oligonucleotides corresponding to mutants SF213-224. The sequences of these oligonucleotides are available on request. All mutants were checked by dideoxy sequencing and are listed in Table 1 .