Arabinosyl transferase responsible for the polymerization of arabinose into the arabinan of arabinogalactan. Positively regulated by the transcriptional regulatory protein EmbR. This is one of the targets of the anti-tuberculosis drug ethambutol [(S,S')-2,2'-(ethylenediimino)di-1-butanol; EMB]. EMB is a first-line drug used to treat tuberculosis. EMB inhibits the transfer of arabinogalactan into the cell wall. Belongs to the emb family.