The Secondary Acceptor A,

com ponent designated Aj still rem ain s a controversial subject. A lthough Aj is believed to be a quinone molecule m ediating redox in teractio n s betw een Aq an d iro n -su lp h u r centre F e - S x , exact model of th e electron tra n s fe r p ath w ay h as y et to be established.

Illu m in atio n a t 200K u n d e r reducing conditions gives rise to a n asym m etric E PR signal 1.03 mT wide centred a t g = 2.0040 co n sisten t w ith a reduced quinone species (Bonnerjea and E vans, 1982). UV absorbance sp ectra an d low tem p era tu re flash induced absorption changes indicate th a t th is species is probably a phylloquinone (V itam in K -1) radical (M ansfield an d E vans, 1985; B rettel et al.y 1986.).

The presence of quinone molecules is confirm ed by solvent extraction w hich in d icate th a t native photosystem -I reaction centre p rep a ratio n s contain two molecules of phylloquinone per P700 (M alkin, 1986). Removal of one of th ese by relatively m ild solvent extraction of freeze dried photosystem -I does not affect electron tra n sfe r (M alkin, 1986; S é tif et al., 1987) b u t hexane-m ethanol extraction of both quinones re su lts in th e blockage of electron tra n sp o rt p a st Aq (Itoh et al., 1987; S é tif et al., 1987; M ansfield et al., 1987^). U nder th ese conditions th e E P R signal assigned to Aj cannot be photo-accum ulated and only th e A^' signal is p re se n t (M ansfield et al., 1987^), although, photoreduction of iro n -su lp h u r centres still occurs a t low tem p era tu re (S étif et al., 1987). E PR experim ents in d icate Fe-Sx is closer to Aq th a n A, an d it h a s been suggested t h a t th is

m ay provide a ro u te for low tem p era tu re photoreduction of iro n -su lp h u r cen tres (Ikegam i an d Itoh, 1987; M ansfield an d E vans, 1988). In Aj

depleted cyanobacterial prep aratio n s, forw ard electron tra n s fe r can be reco n stitu te d by th e addition of phylloquinone, th is resto res both th e m illisecond kinetics and th e ability to reduce NADP^ (Biggins an d M athis, 1988). However, a ran g e of quinones have been rep o rted to rec o n stitu te to th e reaction centre complex (Iwaki and Itoh, 1989) an d th ere h a s been questions over th e specificity of reconstitution.

Photosystem -I sam ples illum inated u n d er high lig h t conditions, in th e presence of dithionite, a t 4°C show P700^ re-reduction kinetics of t% 30 n s indicative of redox centres being blocked beyond Ag. T his h a s been in te rp re te d as th e double reduction of quinone to quinol th ereb y alterin g its m idpoint potential to a value which prevents forw ard electron tra n s fe r to Fe-Sx ai^d confirm ing th e id en tity of (S étif an d B ottin, 1989,1990). Photosystem -I also exhibits an electron spin polarised (ESP) E P R signal th o u g h t to arise from th e P700VA/ radical p a ir (T h u rn a u e r an d G ast, 1985). This signal is lost on both solvent extraction (R u stan d i et al., 1990) a n d double reduction (Snyder et at., 1991) b u t it is reco n stitu te d by re ­ ad d itio n of th e solvent extracted quinone (R ustandi et at., 1990) or re ­ oxidation of th e quinol (Snyder et al., 1991).

All th e evidence th u s far clearly indicates Aj to be a molecule of phylloquinone involved in m ediation of electron tra n s fe r from th e p rim a ry acceptor to F e - S x , however, th ere is contradictory d a ta w hich ren d e rs th e

situ atio n m uch less clear cut.

M ansfield et al. (1987^) found a narrow ing of th e photo-accum ulated Aj E P R signal a fte r photosystem -I particles h ad been exchanged w ith DgO.

In co n trast, cyanobacteria grown in d e u te rated m ethionine, w hich d e u te ra te s m ost of th e phylloquinone, showed no sign of narro w in g in th e E P R signal a ttrib u ta b le to Aj (B arry et al., 1988). Z iegler et al. (1987) an d B iggins et al. (1989) found th a t to tal destruction of b o th m olecules of phylloquinone by irrad iatio n w ith UV light a t 354 nm did n o t affect e ith e r th e photo-accum ulated E PR spectra or th e room te m p e ra tu re photoreduction of benzyl viologen although in activ atio n of NADP^ photoreduction h a s been reported (Biggins an d M athis, 1988). Low te m p e ra tu re photoreduction of iron-sulphur centres w as also u n im peded by photo-inactivation of phylloquinone, although th is m ay be a re s u lt of th e form ation of th e Aq^/Fe-Sx' couple a t cryogenic te m p e ra tu re s (M ansfield a n d E vans, 1988).

T echniques of phylloquinone rem oval or in activ atio n are extrem ely h a rs h an d th e resu lts gained from such tre a tm e n ts m ay b e a r little relevance to the in vivo situation. Cryogenic m easu rem en ts m ay also be m isleading causing anom alies in low tem p eratu re redox events w hich are irre le v a n t a t room tem p eratu re.

A lthough th e m ajority of evidence suggests th a t phylloquinone plays a role in m ediation of forw ard electron tra n sfe r recen t ex p erim en tal d a ta suggests th a t th e situ atio n is not as stra ig h t forw ard a s w as originally th o u g h t a n d fu rth e r work is required before its role can be fully defined.