Translation at higher than an optimal level interferes with coupling at anintercistronic junction

Yu, JS Madison-Antenucci, S Steege, DA

Js. Yu et al., Translation at higher than an optimal level interferes with coupling at anintercistronic junction, MOL MICROB, 42(3), 2001, pp. 821-834

61

INGLESE

Article

Microbiology

MOLECULAR MICROBIOLOGY

0950-382X → ACNP

42

3

2001

821 - 834

ISI

0950-382X(200111)42:3<821:TAHTAO>2.0.ZU;2-O

In pairs of adjacent genes co-transcribed on bacterial polycistronic mRNAs, translation of the first coding region frequently functions as a positive factor to couple translation to the distal coding region. Coupling efficien cies vary over a wide range, but synthesis of both gene products at similar levels is common. We report the results of characterizing an unusual gene pair, in which only about 1% of the translational activity from the upstrea m gene is transmitted to the distal gene. The inefficient coupling was unex pected because the upstream gene is highly translated, the distal initiatio n site has weak but intrinsic ability to, bind ribosomes, and the AUG is on ly two nucleotides beyond the stop codon for the upstream gene. The genes a re those in the filamentous phage IKe genome, which encode the abundant sin gle-stranded DNA binding protein (gene V) and the minor coat protein that c aps one tip of the phage (gene VII). Here, we have used chimeras between th e related phage IKe and f1 sequences to localize the region responsible for inefficient coupling. It mapped upstream from the intercistronic region co ntaining the gene V stop codon and the gene VII initiation site, indicating that low coupling efficiency is associated with gene V. The basis for inef ficient coupling emerged when coupling efficiency was found to increase as gene V translation was decreased below the high wild-type level. This was a chieved by lowering the rate of elongation and by decreasing the efficiency of suppression at an amber codon within the gene. Increasing the strength of the Shine-Dalgarno interaction with 16S rRNA at the gene VII start also increased coupling efficiency substantially. In this gene pair, upstream tr anslation thus functions in an unprecedented way as a negative factor to li mit downstream expression. We interpret the results as evidence that transl ation in excess of an optimal level in an upstream gene interferes with cou pling in the intercistronic junction.