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essential. In practice, temllnator of ~iopali~ie synthase gelle or of the CaMV was fused into
the respective chimeric gene.
Selectablr O I I ~ rryorriilg nrarkers: These are essential comporlents of the total
cassette that is to be tr:rrlsferrcd lo llle pla~ii cells, These genes select ihe transfor~lled cells
from that of the untransibrnled. Sorlle of the popular selectable markers arc ant~b~ot~c
markers such as kanamycin, hygromycin, strepto~nycin etc. Other groups of' selcctablc
nlarkers are the ones that co~lfer resistu~lce to herbicides, such as phopbinohtrici~~.
biolophos, glyphosate. dalapllon etc. As noted above this group of selectable nrarkers can
serve a dual pull,ose: to select transfom~i~nts and to render crops resistant lo respective
herbicides (D'Halluin et al., 1992). The third group is ti~verse, including gencs that cause
resistance to high nitrate, 111gIl a~lli~~o ac~d levels (lys~ne or tl~rconine) or amino ac~d
analogues (Schroit, 1995). Most commonly used selectable genc IS the kanamyc~n
rcsista~~ce gcnc ineo~~lyci~l pl~ospl~otra~~sfcrase (~rpfll). This gene producl dctuxilics
an~inoglycoside ;~~~ub~otics such as ka~~alnyci~~, neonlycin, gencticin and paromomyci~i
(Vurdi et al., 1990). The gene Iipt was isoloied f'rom licoli. It codes for hygrornyc~n
phosphotransfrrasc that deroxilies antibiotic hygronlyci~~.
Kcporter ge~~cs are coding sequences that, upon expressloll in the transgenic plant,
provide a clear 111dicat1011 tlliit gc~letic transfor~llat~o~l did take place. They are useful also
for transient expressio~~ experimenis, in wlirch tl~c iransgene is not necessarily integrated
into the lhost gcnolne. Scllrott (1995) rcvicwcd a review of genes used and tllcir assay
methods. blost co~ll~no~lly used genes used, as ~cporters arc the ones that code for CAT,
GUS, Luciferase and Glee11 1:luorescent I'rotein (GFP). Thc assay for the riidA that codcs
for GUS was developed by Jefferson and his associates (Jefferson, 1987). This gene has