THE LARGE GENOME SEGMENT OF DSRNA BACTERIOPHAGE-PHI-6 IS THE KEY REGULATOR IN THE IN-VITRO MINUS AND PLUS STRAND SYNTHESIS

Bacteriophage phi 6 is a double-stranded RNA (dsRNA) virus that has a genome composed of three linear dsRNA segments (L, M, S). These are en capsidated into a dodecahedral procapsid particle consisting of protei ns P1, P2, P4, and P7. Empty preformed procapsids are able to package the plus-sense single-stranded RNA (ssRNA) of each genome segment, to synthesize the corresponding minus strands (''replication'') to form d sRNA segments, and to continue to the plus strand synthesis (''transcr iption'') in which the dsRNA segments are used as templates in product ion of plus-sense ssRMA. In this study, we have investigated the requi rements for the switch-on of minus and plus strand syntheses. We show that there exists an inverse relationship between regulation of the ss RNA packaging and minus strand synthesis. The packaging of single-stra nded I, which has previously been shown to be packaged as the last, is the necessary signal for the onset of the minus strand synthesis. The absolute requirement for plus strand synthesis is minus strand synthe sis of I, but in addition, the minus strand synthesis of m and the pac kaging of s segment are needed for efficient plus strand synthesis. Fu rthermore, the second nucleotide at the 5'-end of each segment regulat es the extent of the transcription.