The presence of codon-anticodon pairs in the acceptor stem of tRNAs.

A total of 1268 available (excluding mitochondrial) tRNA sequences was used to reconstruct the common consensus image of their acceptor domains. Its structure appeared as a 11-bp-long double-stranded palindrome with complementary triplets in the center, each flanked by the 3'-ACCD and NGGU-5' motifs on each strand (D, base determinator). The palindrome readily extends up to the modern tRNA-like cloverleaf passing through an intermediate hairpin having in the center the single-stranded triplet, in supplement to its double-stranded precursor. The latter might represent an original anticodon-codon pair mapped at 1-2-3 positions of the present-day tRNA acceptors. This conclusion is supported by the striking correlation: in pairs of consensus tRNAs with complementary anticodons, their bases at the 2nd position of the acceptor stem were also complementary. Accordingly, inverse complementarity was also evident at the 71st position of the acceptor stem. With a single exception (tRNA(Phe)-tRNA(Glu) pair), the parallelism is especially impressive for the pairs of tRNAs recognized by aminoacyl-tRNA synthetases (aaRS) from the opposite classes. The above complementarity still doubly presented at the key central position of real single-stranded anticodons and their hypothetical double-stranded precursors is consistent with our previous data pointing to the double-strand use of ancient RNAs in the origin of the main actors in translation- tRNAs with complementary anticodons and the two classes of aaRS.

Vestibular navigation directed by the slope of terrain.

Slope of terrain is an important orienting gradient affecting the goal-directed locomotion of animals. Its significance was assessed in experiment 1 by training rats to find in darkness a feeder on the top of a low cone (80-cm base, 0- to 4-cm high). A computerized infrared tracking system monitoring the rat's position in darkness showed that the path length on the cone surface was inversely proportional to cone height. A device allowing continuous generation of slope-guided locomotion was used in experiment 2. This device consists of a 1-m arena, the floor of which can be supported at a point corresponding to the position of one of three equidistant feeders located 17 cm from its center. The arena is inclined by the locomotion of the rat to a plane passing through the elevated (2- or 4-cm) feeder, the rat's center of gravity, and a point at the edge of the arena resting on the floor. The multitude of such planes generated by the rat's locomotion forms the surface of a virtual cone, the top of which is formed by the feeder. Additional path (difference between distance traveled and shortest distance of the animal from the goal at the onset of inclination) is inversely related to the incline of the arena and is a sensitive measure of performance in this type of vestibular navigation.