Plant Science 148 1999 77 – 88
The strategies of plant virus gene expression: models of economy
Gabrie`le Drugeon , Silvio Urcuqui-Inchima, Malgosia Milner
1
, Gress Kadare´, Rosaura P.C. Valle
2
, Ariane Voyatzakis, Anne-Lise Haenni, Jan Schirawski
Institut Jacques Monod,
2
Place Jussieu - Tour
43
,
75251
Paris Cedex
05
, France Received 27 May 1999; received in revised form 22 June 1999; accepted 24 June 1999
Abstract
Given the small size of their genome, the genetic information of viruses is extremely compact, and non-coding regions are very limited as compared to those of prokaryotic and eukaryotic cell systems. Viruses utilize cell components at all levels of the
replication cycle for their own benefit, not the least being the translation machinery. They have also evolved a number of highly sophisticated strategies to produce and regulate the production of the proteins required for their propagation. In addition, these
proteins are often multifunctional, encoding several essential virus-specific proteins. At the level of transcription, these strategies include splicing, the production of subgenomic RNAs from virus templates and cap-snatching. At the level of translation,
regulation exists at all steps: initiation, elongation and termination. Furthermore, viruses frequently resort to co- andor post-translational cleavage of a polyprotein precursor to yield the mature proteins. © 1999 Published by Elsevier Science Ireland
Ltd. All rights reserved.
Keywords
:
Regulation of protein synthesis; Plant viruses; Post-transcriptional regulation; Gene expression www.elsevier.comlocateplantsci
1. Introduction
For over a century — that is, ever since their discovery — viruses have been a constant source
of new and amazing information and have been determinant in helping us unravel mechanisms
used not only by the viruses themselves, but also by cell systems. Some of these mechanisms appear
to be largely the prerogative of viruses, whereas others often also occur in cells. The study of these
mechanisms has been facilitated in the case of viruses because of the small size of their genome,
and consequently of the relative ease with which viral genes can be isolated as compared to cell
genes genera of viruses are based on Ref. [1]. Thus, it is among viruses that many heretofore
unexpected mechanisms were first described rev. in Ref. [2].
The small size of viral genomes explains the extreme compactness of their genetic information.
Viral genes are often not even separated by inter- genic regions but frequently overlap, and whatever
non-coding regions exist, they are generally of prime importance to regulate replicationtranscrip-
tion of the genome, and translation of the viral proteins. Indeed, few sequences in the genome are
devoid of known function.
Abbre6iations
:
CP, coat protein; g, genomic; IRES, internal ribo- some entry site; MP movement protein; ORF, open reading frame;
RdRp, RNA-dependent RNA polymerase; sg, subgenomic; TGB, triple gene block; UTR, untranslated region; VPg, virus protein
genome-linked; BSMV, Barley stripe mosaic hordeivirus; BYDV- PAV,
Barley yellow
dwarf luteovirus
PAV isolate;
CaMV, Cauliflower mosaic caulimovirus; crTMV, crucifer-infecting TMV;
PVX, Potato potexvirus X; RTBV, Rice tungro bacilliform bad- navirus; SBWMV, Soil-borne wheat mosaic furovirus; STNV, Satel-
lite tobacco necrosis necrovirus; TMV, Tobacco mosaic tobamovirus; WDV, Wheat dwarf geminivirus.
Corresponding author. Tel.: + 33-1-44274035; fax: + 33-1- 44273580.
E-mail address
:
drugeonijm.jussieu.fr G. Drugeon
1
Permanent address: Institute of Biochemistry and Biophysics, Polish Academy of Sciences, ul. Pawinskiego 5a, 02-106 Warszawa,
Poland.
2
Present address: Milagen, Inc., 1387 Marina Way South, Rich- mond, CA 94804, USA.
0168-945299 - see front matter © 1999 Published by Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 1 6 8 - 9 4 5 2 9 9 0 0 1 2 3 - 5
The diversity in shape and structure of viruses is immense rev. in Ref. [1]. Viruses can be envel-
oped or non-enveloped. Their genome can be com- posed of single or double stranded DNA or RNA;
it can be monopartite plant and animal viruses or multipartite plant viruses, rarely animal
viruses. In viruses with a single stranded RNA, the genome can be of positive or of negative
polarity, or it can be ambisense.
To date, the vast majority of plant viruses — the topic of the present review — that have been
investigated contain an RNA genome. Conse- quently, it is among such viruses that many strate-
gies of expression were first described and have been the most thoroughly examined. Plant viral
genomes are small, generally ranging from 4 to 15 kb or kbp. They code for four to 12 proteins.
These include proteins involved in RNA replica- tiontranscription, as for example the RNA-depen-
dent RNA polymerase RdRp among RNA viruses or the reverse transcriptase activity in
Caulimoviruses, transport of the infectious agent from cell to cell by the movement protein MP,
encapsidation of the viral genome by the coat protein CP, vector transmission of the virus by a
protein frequently referred to as the helper compo- nent, proteolytic maturation of viral precursor
polyproteins by viral proteinases, and transactiva- tors that facilitate translation of downstream open
reading frames ORFs. As mentioned above, syn- thesis of the viral proteins is ensured by the host
translation machinery.
Recently, other reviews have dealt with the syn- thesis and function of plant virus proteins [3 – 6].
The aim of the present review is to offer an overview of the strategies of expression as found
in plant viruses. For the sake of convenience, four levels are distinguished at which regulation of gene
expression can occur. These are at the level of the genome segments, transcription and translation,
and at the post-translational level.
2. Regulation of gene expression at the level of the genome segments
