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Understanding the process whereby the ribosome translates the genetic code into
protein molecules will ultimately require high-resolution structural
information, and we report here the first crystal structure of a protein from
the small ribosomal subunit. This protein, S5, has a molecular mass of 17,500
and is highly conserved in all lifeforms. The molecule contains two distinct
alpha/beta domains that have structural similarities to several other proteins
that are components of ribonucleoprotein complexes. Mutations in S5 result in
several phenotypes which suggest that S5 may have a role in translational
fidelity and translocation. These include ribosome ambiguity or ram, reversion
from streptomycin dependence and resistance to spectinomycin. Also, a
cold-sensitive, spectinomycin-resistant mutant of S5 has been identified which
is defective in initiation. Here we show that these mutations map to two
distinct regions of the molecule which seem to be sites of interaction with
ribosomal RNA. A structure/function analysis of the molecule reveals
discrepancies with current models of the 30S subunit.
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