A Twist in the Tail of tRNA

	Biochemist Dr. Alan Weiner is interested in how chromosomes pack, replicate, repair, and access information.

Dr. Kozo Tomita, postdoctoral fellow in biochemistry, and Dr. Alan Weiner, chair of biochemistry, have revealed a new twist to the synthesis of transfer RNA (tRNA), an essential component of cellular protein production. Their results were published in the Nov. 9, 2001, issue of Science.

Most genes encode proteins. Within any cell, the ribosome assembles proteins according to the plan delivered from the genes via messenger RNA (mRNA). The protein components -- 20 different amino acids -- are strung together much like beads on a string. The precise order of the amino acids determines how the protein will fold. The three dimensional structure that emerges controls the tasks that the protein can perform within the cell.

Each amino acid is brought to the ribosome attached to the tail of a cellular transfer RNA (or tRNA) molecule. That tail must have the distinctive nucleic acid sequence CCA (where C is cytidine and A is adenosine) for the ribosome to use it. While some tRNAs are synthesized with an intact CCA end, others must acquire this essential tail after synthesis. In all organisms previously studied, the CCA tail is appended by a CCA-adding enzyme that adds CTP (the precursor of C) twice and ATP (the precursor of A) once.

Tomita and Weiner discovered that in certain bacteria, CCA addition is accomplished not by one dual-purpose CCA-adding enzyme but by two more-specialized enzymes. One enzyme adds CC to the tail; the other adds A. This unexpected division of labor provides clues to the diversification of protein activities diversified over evolutionary time and to gene flow between different bacterial species.