DGGE is simplified for a given fragment when the
region of interest lies in a single domain. However,
functional genes such as the nirS, nirK and nosZ genes
have great sequence variation, and melting profiles obtained
in WinMelt showed that all three denitrification
genes had multiple melting domains. Some of the nirS
fragments that were analysed had up to six different
domains. To minimise the effects of these domains and
avoid complete denaturing of the PCR-amplified fragments,
a 33-bp GC-clamp was added to the three reverse
primers, R3cd, R3Cu and nosZ1622R. The introduction
of the GC-clamp did not affect the amplification efficiency
of either the denitrifying strains or the environmental
samples. After amplification, the three different
DGGE methods in our study were optimised regarding
gradient concentration and running time using amplicons
from the denitrifying strains as controls. We initially
experienced problems with multiple bands, but
once the DGGE methods were optimised, only one band
appeared on the gel from each of the pure cultures
without any smear. After the DGGE optimisations,
partial nirK and nosZ genes from denitrifying pure cultures
were clearly separated from each other and covered
the whole gradient, but the nirS-gene separation
was not as good (e.g., Fig. 2).