Catalytic subunit of cellulose synthase. It polymerizes uridine 5'-diphosphate glucose to cellulose, which is produced as an extracellular component for mechanical and chemical protection at the onset of the stationary phase, when the cells exhibit multicellular behavior (rdar morphotype). Coexpression of cellulose and thin aggregative fimbriae (curli fimbrae or fibers) leads to a hydrophobic network with tightly packed cells embedded in a highly inert matrix that confers cohesion, elasticity and tissue-like properties to colonies (PubMed:24097954). [(1->4)-beta-D-glucosyl](n) + UDP-alpha-D-glucose = [(1->4)-beta-D-glucosyl](n+1) + H(+) + UDP Activated by bis-(3'-5') cyclic diguanylic acid (c-di-GMP). Glycan metabolism; bacterial cellulose biosynthesis. Part of the yhjR-bcsQABZC operon. Expressed at low levels in mid-log phase, expression increases as cells enter stationary phase, the increase in stationary phase is dependent on rpoS (PubMed:24097954). Expression is higher at 28 than 37 degrees Celsius (at protein level) (PubMed:24097954). There are two conserved domains in the globular part of the protein: the N-terminal domain (domain A) contains the conserved DXD motif and is possibly involved in catalysis and substrate binding. The C-terminal domain (domain B) contains the QXXRW motif and is present only in processive glycosyl transferases. It could be involved in the processivity function of the enzyme, possibly required for holding the growing glycan chain in the active site. When disrupted in a cellulose-synthesizing strain (a strain K12 / W3110 derivative called AR3110 with a restored, functional bcsQ gene), cellulose is no longer made and the rdar morphotype is lost (PubMed:24097954). The genes bscA, bcsB, bcsZ and bcsC are constitutively transcribed but cellulose synthesis occurs only when DgcC, a putative transmembrane protein regulated by CsgD, is expressed. Cellulose production is abolished in E.coli K12 / MG1655 and W3110 due to a premature stop codon in bcsQ (PubMed:24097954). Belongs to the glycosyltransferase 2 family. Extended N-terminus.