ENHANCED RIBOSOME FRAMESHIFTING IN STATIONARY-PHASE CELLS

We have examined the effect of growth phase in Escherichia coli on the a translation of a plasmid-borne lacZ gene in which active enzyme syn thesis requires a leftward frameshift. During the log phase of growth, the differential rate of enzyme synthesis is very low. It increases b y about two orders of magnitude during the small amount of protein syn thesis which occurs at the end of log phase and the early part of stat ionary pi-case. The increase is sufficient to increase the enzyme's sp ecific activity in crude extracts to 30 times more than it would be if the log-phase differential rate continued unchanged. No such large in crease is observed with a zero-frame lacZ(+) control gene on the same plasmid under the control of the same promoter; a significant but much smaller increase is observed with a zero-frame control containing an in-frame terminator tripler in the region of the required frameshift. Protein sequence analysis of the enzyme made from the frameshift repor ter in stationary cells shows that the increased enzyme synthesis is d ue to frameshifting, and not due to termination and reinitiation. The frameshift occurs at or right after the sequence U UUC AAG, an intrins ically shifty site. (C) 1996 Academic Press Limited