5.3. Acclimation of photosynthesis, pigments and phycobilisome assembly during and after six months of darkness - simulating
5.3.2. The influence of darkness On phycobilisome assembly
The influetzce of darktzess otz the atnoz~izt of PBSILp und PBSlo,,.
Acclinzatioti to darktiess. No alteration in the total phycobilisome amount and in the PBS:PSII:PSI stoichio~net~y was Seen in the first three months o f darkness, selated from almost constant APC tissue contents and constant APC:Chl ci ratios. A decsease in both pasameters to minimal values occussed in the last three months o f daskness and was accompanied by the strong seduction in F,/F,,, and in photosynthetic perfosmance.
One intesesting result o f the psesent study is, that PBSlo,+ disappeased after two months o f daskness. Since no decsease in the total amount o f phycobilisomes and in the phycobiIipsotein tissue content was seen, a conversion fsom PBS,,),, into PBSup is possible, like pseviously discussed fos the other direction in the long-te~m expenment without daskness. This suppo~-t the theory, that PBSup function as basic light hasvesting antennae, probably to build up PBSI,,*, when an enhancement in photosynthetic pesformance is sequired. However, the semaining PBSUp decseased in its amount in the last three months o f daskness.
Accli~~zutioti to re-illi~t~zi~zat~otz. Afies one month o f se-exposure to light, the total phycobilisome number was secovesed, and was maximal in Decembes, resulting in maximal APC:Chl a ratios. The time scale study showed, that PBS,<,,+ appeased fisst after 24 days o f se-illumination (data not shown). During the summer months the total
phycobilisome n m b e r decreased again. PBSlOw appeared only in small aInoilnts arid was almost negligible in Februwy, This agree with the finding in the long-term experiment without darkness, that mainly ttle anlount of PBSiOw decreased from December onwards during the Antarctic suminer n~onths.
The i~zfluetzce of dark~zess otz the size of PBSrLP urzd PBSlo,,,
Acclittzuiio~~ io durkt~ess. The size of PBS,,,, and PBStoq, semain unaltered during darkness, selated fsom mose or less constant ratios of PE:APC, PC:APC and PE:PC.
Only in the last rnonth of darkness, PBSup clearly loosed rods, which are consistent of PC and PE. The strong increase in the PE:PC ratio suggests, that the remaining rods are mainly consistent of PE.
A c c l i ~ ~ ~ c l t i o ~ ~ io re-ill~~t~~itzutiotz. After one month of re-illumination, the phycobilisome size of PBSUp was recovered totally, 2nd a further coupling of PE occut-red. In contrast, PBS,c>,v incseased clearly in size one month later, rnainly by coupling of PE. The PC content was not fully recovered. However, maximal phycobilisome size was achieved between Decernbes and February, both in PBSup and PBSlOw. The PE:APC ratios were even so~newhat higher than the values measured in the Iong-ternl experiment without daskness.
The ii~flueizce of darkrzess orz the rods (Y-subu~zits, LR 38.9PC J a ~ ~ d L ~ ~ ~ ~ ' ~ ~ ~ ) Acclit~zutio~~ to d u r k ~ ~ e s . ~ . The alteration in the phycobilisome size was Seen more
clearly in the seasonal changes of the percentage ratios (%-ratios) of the linker
3 8 . 9 K 30.61T
polypeptides belonging to the rods (Y-subunits, LR and LRc 1. In the first fous months of darkness in PBSUp. the proportions of all linkers were nlore or less the Same, indicating no alteration in the phycobilisome size. Even PBSIo5, showed no changes in the proportions of all linkers during the single month of darkness. However, in the last two months of darkness, in PBSUL, the amount of the rod core linker L~~~~~~~ and one month later of the rod linkes LR3' and the y4' j-subunit clearly decreased. indicating a loss of rods containing PC and PE. Consequently, the remaining T''-subunit increased in its relative propostion. In contrast, the
y3
'-siibunit seems to be unaffected during darkness.This agrees with the finding in the Iong-tem expenment without darkness, that the smallest phycobilisome size was characterised by the smallest amount of the
y3
'-subunit, found in April just before darkness begun. Since no increase in phycobilisome size was recorded during darkness, even no increase in the amount of the y3"-subunit is suspected.Discussion 125
The degradation of phycobilisomes was described in N- and S-depxived cells of cyanobacteria as an ordered process (Yamanaka and Glazer 1980, Collier and Grossman 1992, Grossman 1994). The degsadation started at the periphery of the phycobiIisomes with the elimination of the terminal hexamer and its associated linkes polypeptide, followed by degradation of the next hexamer and its associated linker polypeptide and subsequently, by the loss of some entire rods. This 'trimming' process was generally complete within 24 hours. The degsadation of phycobilisome polypeptides sesulted in a decsease in the phycobilisome size and a reduction in the satio of PC:APC. The smalles phycobilisomes still functioned in harvesting light enesgy. Continued nutrient deprivation sesulted in the complete degradation of the remaining phycobilisome structuse. Upon adding the limiting nutrient back to the deprived cultuses, new phycobilisomes x e rapidly synthesised to normal levels before growth resumes. The phycobilisome degsadation seems to be catalysed by a Protease, that is synthesised de novo in response to N- and S- deprivation. The increased expression of the small gene rzblA, found in a 2 kbp fragment of genomic DNA, triggers the phycobilisome degradation (Collier and Grossman 1994).
Accli~natiotz to re-illi~r~zitzatiorz. After one rnonth of re-exposure to light, in PBSup LR:0.6pc, LR389pcand yo3-subunit wese recovesed and a strong increase in the amount of the y35-subunit was evident, indicating an increase of the phycobilisome size by coupling of the y3'-submit and its associated PE hexanler, like previously observed in the long- term experiment without darkness,
After re-illumination, even in PBSio,v a strong increase in the a~nount of the $35-subunit with maximal values in December occurred, but the LRc30hp(' and probably also the L~~~~~~ were not secovered before February, This supports the theory, that the yj3'- subunit and its associated PE hexamer might form rods consistent only of PE. ~vhich might directly couple to APC as previously discussed in the long-tenn experiment without darkness.
Howeves, both in PBSUp and PBSi,,,v an uncoupling of some $35-subunit and its accociated PE hexamer was noticed in the Antarctic Summer months, as previously obsesved in the long-tem expenment without daskness. Nevestheless, the setnaining amount of the
Y3
'-subunit was still extsemly high and reached values comparable to the maximal values measured in the long-tem experinient without darkness. This might explain, why no strong changes in the PE:APC satios in January and Febsuasy were Seen.However, a clear seduction was Seen in March.
The iizflueizce of darkizess oiz the core (Lclw atzd Lc)
Anothes interesting point is the changes of the two lasge core membsane linkes L ~and ~ ~ ~ ' ~
75.4
LcM
.
Befose and dusing daskness, 'sn~all' phycobilisomes wese chasacterised by a highes abundance of the smalles LCM7' 4, both in PBSup and PBSlOw. After se-illumination, the lasge phycobilisomes found in Janualy and Februasy wese chasacte~ised b y a highes amount of the lasges k M 8 ' . 0 and a lowes amount of the L ~ ~ : ~ . ~ . This agsees with the finding in the long-tesm expesiment without daskness, and supposts the t h e o ~ y of an acclirnation OS segulation pheno~nena of the phycobilisome core as previously discussed.The second linkes type involved in the assembly of the core, the small core linker L ~ " , was psesent Inose or less in the Same abundance oves the entise yeas. Like previously observed in the long-tesm expesiment without daskness, a second low molecula mass polypeptide of 8.0 kDa was psesent only in the Esst past of the yeas, and was suspected to be a second core linker.
The iizflueizce of darkizess O I Z coiztat?ziizaizts
As n~entioned above, the sen~aining polypeptides wese often found in SDS gels of phycobilisornes. The 57.9 kDa and 52.0 kDa polypeptides, which incseased cleasly in the last two months of daskness, might be psoteases sesponsible fos the degsadation of the phycobisomes.