PCR analysis of selected gradient fractions. PCR amplification of cDNA reverse-transcribed from bacterial 16S rRNA harvested from gradient fractions 1–7 in a PCR assay resulted in an ~550 bp fragment with increased amounts of amplicons in “heavy” RNA-SIP fractions for the 2 h and 4 h incubations with [U13C]glucose. The fractions covered a BD spectrum from 1.84 g mL−1 (fraction 1) to 1.796 g mL−1 (fraction 7). The picture is combined from three stained agarose gels after electrophoresis of the 16S RNA amplicons. For simplification, the size standard is not shown. All gels contained the same volume of PCR mixture. Faint bands in fractions 3 (uncultured control, 0 h) and 2 (2 h and 4 h 13C-cultures) represent low amounts of RNA amplicons which were not suitable for further analyses.