An updated sul1 gene sequence database was constructed and new degenerate primers were designed to better investigate the abundance,diversity,and variation of a ubiquitous antibiotic resistance gene,sul1,with PCR-based methods in activated sludge from wastewater treatment plants (WWTPs).Compared to previous sul1 primers,the newly designed degenerate primers showed higher coverage in both in-silico evaluations and activated sludge samples.Using the new primers,the abundance and diversity of sul1 gene,together with 16S rRNA gene,in activated sludge from five WWTPs in summer and winter were determined by quantitative PCR and MiSeq sequencing.The sul1 gene was found to be prevalent and displayed a comparable abundance (average 8.06%) to the total bacterial cells across all samples.However,compared to the significant temporal and geographical divergences in the quantity and diversity of bacterial communities in WWTPs,there were no significant temporal or spatial variations of representative OTUs ofsull gene in most cases.Additionally,the representative sul1 OTUs showed fairly close phylogeny and there was no obvious correlation between them and the dominant bacterial genera,suggesting that mobile genetic elements likely play a greater role than specific microbial taxafor sul1 gene in WWTPs,and a mobile component containing the sul1 gene undergoes frequent transfer among different bacterial species without any stable hosts.