cDNAs encoding three isoforms of OGT (ncOGT, mOGT, and sOGT) were expressed in Escherichia coli in which the coexpression system of OGT with target substrates was established in vivo. No endogenous bacterial proteins were significantly O-GlcNAcylated by any type of OGT isoform while co-expressed p62 and Sp1 were strongly O-GlcNAcylated by ncOGT. These results suggest that most of bacterial proteins appear not to be recognized as right substrates by mammalian OGT whereas cytosolic environments may supply UDP-GlcNAc enough to proceed to O-GlcNAcylation in E. coli. Under these conditions, sOGT was auto-O-GlcNAcylated whereas ncOGT and mOGT were not. Importantly, we found that when Sp1 was coexpressed, ncOGT can O-GlcNAcylate not only Sp1 but also many bacterial proteins. Our findings suggest that Sp1 may modulate the capability of target recognition of ncOGT by which ncOGT can be led to newly recognize bacterial proteins as target substrates, finally generating the O-glyco-bacteria. Our results demonstrate that the O-glyco-bacteria showed enhanced thermal resistance to allow cell survival at a temperature as high as 52 degrees C. (C) 2008 Elsevier Inc. All rights reserved.