Terminal redundancy of 'High frequency of recombination' markers of Bacillus subtilis phage SPO1

Mutations in 2 of the 36 cistrons known for Bacillus subtilis phage SPOT were unmappable because they showed high frequencies of recombination with each other and with mutations in all other cistrons (Okubo, S., Yanagida, T., Fujta, D. J., and Ohlsson-Wilhelm, B. M., 1972, Biken J. 15 , 81–97). These cistrons (35 and 36) are among the last to be replicated and their replication is specifically inhibited by mutants affected in SPOT cistron 32 (Glassberg, J., Franck, M., and Stewart, C. R., 1977a, J. Virol. 21 , 147–152). We propose that these observations can be explained by the location of these two cistrons within a region of terminal redundancy, and we present the following data in support of this proposal: (1) Mixed infection between sus 35 + and sus 35 − phage yields bursts which include a high proportion of heterozygotes. (2) The sus 35 genetic marker is located on two different restriction fragments. (3) By restriction nuclease mapping, one copy of this marker has been located to the right of the rightmost markers on the known genetic map. (4) Two of the fragments produced by Eco RI ∗ digestion of mature DNA are not found in digests of intracellular DNA, and no new fragment is found, suggesting that these fragments are part of a region of terminal redundancy used for the formation of concatemers. One of these fragments carries the sus 35 marker.