in rabies - libdoc.who.int - World Health Organization

LABORATORY TECHNIQUES IN RABIES
One of the main characteristics of the L protein is the sequence homology that
it shows with the L protein of other members of the Mononegavirales order which
suggests that they have evolved from a common ancestor (40 73) The homology
is not randomly distributed along the protein however and some domains are
highly conserved with the amino acids in identical positions wh~le others are more
variable Such a distiibution of independent conserved domains joined by more
variable areas is consistent w~th the multifunctional nature of the L protein (40, 73)
It suggests that different activities are linked within the final polypeptide where
they may retain a certain degree of independence, as indicated by the comple
mentation observed between VSV L protein mutants Several activities were tentatively
attributed to certain domains providing the first available guidelines for the
future dissection of the functions of the L protein by site-directed mutagenesis (73)
It is noteworthy that the key domain catalysing the polymerization of the rabies
genome (' polynierase module ) has been mapped The ' polymerase module
appears ubiquitous in the animal kingdom since it is similar in all the RNAdependent
polymerases (RNA polymerases and reverse transcriptases) and partly
conserved in the DNA-dependent polymerases (74 75) These results illustrate
how rabies research can have influence beyond its own domain
Genomic signals
The start and stop transcription sigrials (internal signals) flanking the cistrons of
the rabies and Mokola genome have been determined by S1 nuclease mapping
experiments (6,39) They are composed of 9 nucleotide long consensus
sequences closely related to those of VSV The stop signal is terminated by
a sequence of 7 uridine residues which are reiteratively copied by the transcriptase
in order to produce the polyadenylation tail of each mRNA before re-initiating at
the next start signal
The promoters for polymerization and for encapsidation (external signals) are
assumed to be present in the first 11 nucleotides of the 3'- and 5'-ends of the
genome respectively (40 72 76) These sequences are strictly conserved and are
inversely coniplementary However the genome is not likely to generate a panhandle
structure during transcription or replication since the genornes and
antigenomes are encapsidated as soon as they are synthesized
Within a viral genus (Lyssawriis or Ves~culov~rus) all the signals are strongly
conserved However when the whole Rhabdoviridae family is considered the
internal signals appear more stable than the external ones (Fig 3 7)
The intergenic regions
The intergenic regions of the rabies and Mokola virus genomes (Fig. 3.7) vary both
in length (2-24 nucleotides) and nucleotide composition (l, 39, 72), while those of
the VSV genome (Indiana strain) are conserved (mostly GA). This variation could
modify the progressive loss of transcriptional efficiency occurring between cistrons,
and partly explain the weaker performance of rabies virus compared with
VSV during cell infection. slower cycle of transcription and replication, less
production, and virtually no inhibition of cellular synthesis.