Part of the relaxosome complex that is responsible for plasmid transfer during conjugation. Locally unwinds DNA and catalyzes the cleavage of one of the DNA strands at oriT. The cleaved strand is then transferred through the dedicated type IV secretion apparatus. MobA remains covalently linked at the 5' end of the strand, and once in the recipient cell, it probably catalyzes the rejoining of the two ends of the strand, re-forming the circular plasmid DNA. The primase activity of MobA is essential for the synthesis of primers that will initiate the DNA replication events necessary to form the double-stranded plasmid in the recipient cell. ATP-independent breakage of single-stranded DNA, followed by passage and rejoining. ssDNA + n NTP = ssDNA/pppN(pN)n-1 hybrid + (n-1) diphosphate. Divalent metal cation. Can use Mg(2+), or to a lesser extent, Mn(2+), Ca(2+) or Ba(2+). Interacts with MobB and MobC to form the relaxosome. Belongs to the MobA/MobL family.