Involved in the biosynthesis of the mycolylarabinogalactan-peptidoglycan (mAGP) complex, an essential component of the mycobacterial cell wall. Catalyzes the transfer of the rhamnosyl moiety from dTDP-rhamnosyl (dTDP-Rha) onto the decaprenyl-pyrophosphoryl-GlcNAc (C50-PP-GlcNAc), yielding rhamnosyl-decaprenyl-pyrophosphoryl-GlcNAc (Rha-C50-PP-GlcNAc). dTDP-beta-L-rhamnose + N-acetyl-alpha-D-glucosaminyl-1-diphospho-trans,octa-cis-decaprenol = alpha-L-rhamnosyl-(1->3)-N-acetyl-alpha-D-glucosaminyl-diphospho-trans,octa-cis-decaprenol + dTDP + H(+) Optimum pH is 8.6. Optimum temperature is 30 degrees Celsius. Cells lacking this gene lead to irreversible loss of viability. Was identified as a high-confidence drug target. Belongs to the glycosyltransferase 2 family. Truncated N-terminus.