In vitro, a bacteriophage T4 primase composed of T4 61 and 41 proteins, catalyzes the formation of pentaribonucleotides used to initiate DNA synthesis on single-stranded DNA. We have determined that cells containing a plasmid with the T4 DNA from 18.68 to 15.05 map units express an activity that substitutes for authentic 61 protein in vitro in catalyzing primer-dependent DNA synthesis with six other T4 DNA replication proteins. This result establishes that this region, genetically assigned to gene 61, is the structural gene for the priming protein. Cells containing a plasmid with gene 61 downstream of the strong phage lambda promoter PL and the antitermination site nutL produce 100-fold more 61 protein than T4-infected cells. We have developed an improved purification procedure which yields 100 mg of homogeneous, active protein from 178 g of these cells. In the plasmid, the T4 DNA downstream of gene 61 expresses a protein of 30,000 daltons. This protein may be the T4 DNA adenine methylase (dam) gene product, since Schlagman and Hattman (Schlagman, S. L. and Hattman, S. (1983) Gene 22, 139-156) have shown that this activity is expressed by plasmids containing T4 DNA from this region. In the PL, nutL vector, the expression of both the 30,000-dalton and 61 proteins is enhanced up to 20-fold by the presence of the phage lambda N protein, a transcription antitermination protein, suggesting that expression of the T4 DNA in the plasmid may be regulated transcriptionally. In addition, in both N+ and N- cells, the level of 61 protein, whose gene is proximal to PL on the plasmid, is lower than that of the product of the promoter distal 30,000-dalton protein gene. This result suggests that, at least in the plasmid construction, the expression of 61 protein may also be regulated after transcription.