Transfers mannose from Dol-P-mannose to Ser or Thr residues on proteins (By similarity). Involved in DNA conjugation, in at least the recipient strain (PubMed:18554329). a dolichyl beta-D-mannosyl phosphate + L-seryl-[protein] = 3-O-(alpha-D-mannosyl)-L-seryl-[protein] + a dolichyl phosphate + H(+) a dolichyl beta-D-mannosyl phosphate + L-threonyl-[protein] = 3-O-(alpha-D-mannosyl)-L-threonyl-[protein] + a dolichyl phosphate + H(+) Protein modification; protein glycosylation. Loss of DNA conjugation when disrupted in recipient strain, strain does not secrete EsxB (PubMed:18554329). DNA conjugation in M.smegmatis is unidirectional with distinct donor and recipient strains; mc(2)155 is a donor strain while MKD8 is a recipient strain. Mutations in a donor strain that alter DNA transfer do not always alter DNA transfer in a recipient strain. Belongs to the glycosyltransferase 39 family. Truncated N-terminus.