PATTERNS OF CONVERGENT AND OVERLAPPING TRANSCRIPTION WITHIN B2-REGION OF COLIPHAGE LAMBDA

The central b2 region in the conventional vegetative map of the coliphage λ genome was found to be a unique DNA segment in which two opposingly oriented mRNA syntheses, one originating on the r strand of the left arm and the other on the l strand of the right arm, converge and partially overlap (Figs. 1 and 2). Thus, a segment located to the left of the center of the b2 region, for which no functional genes have been found, appears to act as a buffer zone between the two converging transcriptions, some complementary RNA sequences being produced in the region of overlapping syntheses. The l strand of the b2 region is transcribed early and late after infection or induction. However, there is little or no such transcription upon induction of singly lysogenized bacteria when the excision and circularization of λ are blocked. It appears, therefore, that the l strand transcription of the b2 region does depend on coupling between b2 and the early N-a′ genes; this link is characteristic of the DNA extracted from phage particles and of the circularized genomes, but is disrupted in the singly lysogenic form of λ, which is cyclicly permuted (Fig. 1). The r strand within the left segment of the b2 region becomes transcribed only late after infection or induction, as if it were coupled to the A-J genes, which are expressed late and transcribed from strand r. When protein synthesis is inhibited by chloramphenicol or when gene N is nonfunctional, the b2 region is not transcribed from either strand of DNA. The possible roles of the b2 region, which constitutes a model system for studying convergent transcriptions, are discussed. © 1969.