Flightin is a myosin rod binding protein

The assembly of striated muscle myosin into thick filaments of precise and regular length requires the assistance of accessory proteins. Drosophila indirect flight muscle (IFM) contain flightin, a 20-kDa protein that has been shown to be essential for flight, for maintenance of sarcomeric integrity in active muscle, and informative in length determination of thick filaments during IFM development. Additionally, a point mutation in the myosin rod (Mhc ¹³) negates flightin accumulation in the IFM in vivo. The manner in which flightin interacts with thick filaments is not known. Here, two different solid-state binding assays demonstrate that flightin binds to myosin and to a recombinant fragment of the myosin rod that include the COOH-terminal 600 amino acids (zone 19 to tail piece). The interaction of flightin and myosin is abolished by the single amino acid substitution in Mhc ¹³ at position 1e of zone 27 of the red (residue 1554). The molar ratio of flightin to myosin is approx 1∶1 to 1∶2. Thus, the instability of thick filaments, seen in vivo in the absence of flightin suggests that the flightin-myosin interaction is critical for maintaining sarcomere integrity in active muscle.     

Flightin Is Necessary for Length Determination, Structural Integrity, and Large Bending Stiffness of Insect Flight Muscle Thick Filaments

Despite the fundamental role of thick filaments in muscle contraction, little is known about the mechanical behavior of these filaments and how myosin-associated proteins dictate differences between muscle types. In this study, we used atomic force microscopy to study the morphological and mechanical properties of fully hydrated native thick filaments isolated from indirect flight muscle (IFM) of normal and mutant Drosophila lacking flightin (fln⁰). IFM thick filaments from newly eclosed (0-1 h old) wild-type flies have a mean length of 3.04 ± 0.05 μm. In contrast, IFM thick filaments from newly eclosed fln⁰ flies are more variable in length and, on average, are significantly longer (3.90 ± 1.33 μm) than wild-type filaments from flies of the same age. In the absence of flightin, thick filaments can attain lengths > 300% of wild-type filaments, indicating that flightin is required for setting the proper filament length in vivo. Filaments lacking flightin are structurally compromised, and filament preparations from fully matured 3- to 5-day-old adult fln⁰ IFM yielded fragments of variable length much shorter than 3.20 ± 0.04 μm, the length obtained from wild-type flies of similar age. The persistence length, an index of bending stiffness, was calculated from measurements of filament end-to-end length and contour length. We show that the presence of flightin increases persistence length by more than 40% and that wild-type filaments increase in stiffness with age. These results indicate that flightin fulfills an essential role in defining the structural and mechanical properties of IFM thick filaments.     

Flightin, a novel myofibrillar protein of Drosophila stretch-activated muscles

The indirect flight muscles of Drosophila are adapted for rapid oscillatory movements which depend on properties of the contractile apparatus itself. Flight muscles are stretch activated and the frequency of contraction in these muscles is independent of the rate of nerve impulses. Little is known about the molecular basis of these adaptations. We now report a novel protein that is found only in flight muscles and has, therefore, been named flightin. Although we detect only one gene (in polytene region 76D) for flightin, this protein has several isoforms (relative gel mobilities, 27-30 kD; pIs, 4.6-6.0). These isoforms appear to be created by posttranslational modifications. A subset of these isoforms is absent in newly emerged adults but appears when the adult develops the ability to fly. In intact muscles flightin is associated with the A band of the sarcomere, where evidence suggests it interacts with the myosin filaments. Computer database searches do not reveal extensive similarity to any known protein. However, the NH2-terminal 12 residues show similarity to the NH2- terminal sequence of actin, a region that interacts with myosin. These features suggest a role for flightin in the regulation of contraction, possibly by modulating actin-myosin interaction.     

COOH-terminal truncation of flightin decreases myofilament lattice organization, cross-bridge binding, and power output in Drosophila indirect flight muscle

The indirect flight muscle (IFM) of insects is characterized by a near crystalline myofilament lattice structure that likely evolved to achieve high power output. In Drosophila IFM, the myosin rod binding protein flightin plays a crucial role in thick filament organization and sarcomere integrity. Here we investigate the extent to which the COOH terminus of flightin contributes to IFM structure and mechanical performance using transgenic Drosophila expressing a truncated flightin lacking the 44 COOH-terminal amino acids (fln(ΔC44)). Electron microscopy and X-ray diffraction measurements show decreased myofilament lattice order in the fln(ΔC44) line compared with control, a transgenic flightin-null rescued line (fln(+)). fln(ΔC44) fibers produced roughly 1/3 the oscillatory work and power of fln(+), with reduced frequencies of maximum work (123 Hz vs. 154 Hz) and power (139 Hz vs. 187 Hz) output, indicating slower myosin cycling kinetics. These reductions in work and power stem from a slower rate of cross-bridge recruitment and decreased cross-bridge binding in fln(ΔC44) fibers, although the mean duration of cross-bridge attachment was not different between both lines. The decreases in lattice order and myosin kinetics resulted in fln(ΔC44) flies being unable to beat their wings. These results indicate that the COOH terminus of flightin is necessary for normal myofilament lattice organization, thereby facilitating the cross-bridge binding required to achieve high power output for flight.     

Suppression of muscle hypercontraction by mutations in the myosin heavy chain gene of Drosophila melanogaster

The indirect flight muscles (IFM) of Drosophila melanogaster provide a good genetic system with which to investigate muscle function. Flight muscle contraction is regulated by both stretch and Ca(2+)-induced thin filament (actin + tropomyosin + troponin complex) activation. Some mutants in troponin-I (TnI) and troponin-T (TnT) genes cause a "hypercontraction" muscle phenotype, suggesting that this condition arises from defects in Ca(2+) regulation and actomyosin-generated tension. We have tested the hypothesis that missense mutations of the myosin heavy chain gene, Mhc, which suppress the hypercontraction of the TnI mutant held-up(2) (hdp(2)), do so by reducing actomyosin force production. Here we show that a "headless" Mhc transgenic fly construct that reduces the myosin head concentration in the muscle thick filaments acts as a dose-dependent suppressor of hypercontracting alleles of TnI, TnT, Mhc, and flightin genes. The data suggest that most, if not all, mutants causing hypercontraction require actomyosin-produced forces to do so. Whether all Mhc suppressors act simply by reducing the force production of the thick filament is discussed with respect to current models of myosin function and thin filament activation by the binding of calcium to the troponin complex.     

Alterations in flightin phosphorylation inDrosophila flight muscles are associated with myofibrillar defects engendered by actin and myosin heavy-chain mutant alleles

Flightin is a 20-kD myofibrillar protein found in the stretch-activated flight muscles ofDrosophila melanogaster. Nine of the eleven isoelectric variants of flightin are generatedin vivo by multiple phosphorylations. The accumulation of these isoelectric variants is affected differently by mutations that eliminate thick filaments or thin filaments. Mutations in the myosin heavy-chain gene that prevent thick filament assembly block accumulation of all flightin variants except N1, the unphosphorylated precursor, which is present at much reduced levels. Mutations in the flight muscle-specific actin gene that block actin synthesis and prevent thin filament assembly disrupt the temporal regulation of flightin phosphorylation, resulting in premature phosphorylation and premature accumulation of flightin phosphovariants. Cellular fractionation of fibers that are devoid of thin filaments show that flightin remains associated with the thick filamentrich cytomatrix. These results suggest that flightin is a structural component of the thick filaments whose regulated phosphorylation is dependent upon the presence of thin filaments.This work was supported by National Science Foundation Grant IBN-9253045.     

Flightin is essential for thick filament assembly and sarcomere stability in Drosophila flight muscles

Flightin is a multiply phosphorylated, 20-kD myofibrillar protein found in Drosophila indirect flight muscles (IFM). Previous work suggests that flightin plays an essential, as yet undefined, role in normal sarcomere structure and contractile activity. Here we show that flightin is associated with thick filaments where it is likely to interact with the myosin rod. We have created a null mutation for flightin, fln(0), that results in loss of flight ability but has no effect on fecundity or viability. Electron microscopy comparing pupa and adult fln(0) IFM shows that sarcomeres, and thick and thin filaments in pupal IFM, are 25-30% longer than in wild type. fln(0) fibers are abnormally wavy, but sarcomere and myotendon structure in pupa are otherwise normal. Within the first 5 h of adult life and beginning of contractile activity, IFM fibers become disrupted as thick filaments and sarcomeres are variably shortened, and myofibrils are ruptured at the myotendon junction. Unusual empty pockets and granular material interrupt the filament lattice of adult fln(0) sarcomeres. Site-specific cleavage of myosin heavy chain occurs during this period. That myosin is cleaved in the absence of flightin is consistent with the immunolocalization of flightin on the thick filament and biochemical and genetic evidence suggesting it is associated with the myosin rod. Our results indicate that flightin is required for the establishment of normal thick filament length during late pupal development and thick filament stability in adult after initiation of contractile activity.     

Heat shock protein 20-mediated force suppression in forskolin-relaxed swine carotid artery

Increases in cyclic nucleotide levels induce smooth muscle relaxation by deactivation [reductions in myosin regulatory light chain (MRLC) phosphorylation (e.g., by reduced [Ca²⁺])] or force suppression (reduction in force without reduction in MRLC phosphorylation). Ser¹⁶-heat shock protein 20 (HSP20) phosphorylation is the proposed mediator of force suppression. We evaluated three potential hypotheses whereby Ser¹⁶-HSP20 phosphorylation could regulate smooth muscle force: 1) a threshold level of HSP20 phosphorylation could inactivate a thin filament as a whole, 2) phosphorylation of a single HSP20 could fully inactivate a small region of a thin filament, or 3) HSP20 phosphorylation could weakly inhibit myosin binding at either the thin- or thick-filament level. We tested these hypotheses by analyzing the dependence of force on Ser¹⁶-HSP20 phosphorylation in swine carotid media. First, we determined that swine HSP20 has a second phosphorylation site at Ser¹⁵⁷. Ser¹⁵⁷-HSP20 phosphorylation values were high and did not change during contractile activation or forskolin-induced relaxation. Forskolin significantly increased Ser¹⁶-HSP20 phosphorylation. The relationship between Ser¹⁶-HSP20 phosphorylation and force remained linear and was shifted downward in partially activated muscles relaxed with forskolin. Neither forskolin nor nitroglycerin induced actin depolymerization as detected using the F/G-actin ratio method in smooth muscle homogenates. These results suggest that force suppression does not occur in accordance with the first hypothesis (inactivation of a thin filament as a whole). Our data are more consistent with the second and third hypotheses that force suppression is mediated by full or partial inhibition of local myosin binding at the thin- or thick-filament level. cAMP; cGMP; nitric oxide; vascular smooth muscle     

Molecular and functional characterization of the flightin gene in the Oriental fruit fly,Bactrocera dorsalis

Flightin is a protein in flight muscles and is crucial for the flight capacity. Flightin also has been proposed as a protein with deep ancestry and functions outside of flight muscles. However, functional and molecular characterization of flightin achieved so far is mainly in flight muscles of Drosophila. Here, we cloned the flightin (Bd-flightin) gene and tested its expression and function in Bactrocera dorsalis, an important migratory pest. Phylogenetic analysis based on flightin orthologs revealed that the divergence of flightin is consistent with the taxonomic classification of insects. Motif analysis indicated obvious variations in flightin orthologs, which may have occurred during speciation and functional differentiation. The expression is quite low during egg and larval stages, which largely increased during pupal stage and then peaked at the beginning of the adult stage. Bd-flightin also showed tissue- and age-specific expression patterns during adult stage. The relative expression level is low in wing, head, ovary and testis, which is relatively higher in leg and abdominal wall and much higher in thorax. Injection of late pupae and newly eclosed adults with 1 μg flightin dsRNA per insect both significantly reduced the expression of flightin and the flight capacity in males and females. In addition, silencing the expression of flightin also decreased the weight ratio of thorax and whole-body. These results suggested that flightin plays important roles in flight muscle development and flight function in B. dorsalis, which can potentially be used to control the flight behaviour of the fruit fly.     

Phosphorylation-dependent power output of transgenic flies: an integrated study

We examine how the structure and function of indirect flight muscle (IFM) and the entire flight system of Drosophila melanogaster are affected by phosphorylation of the myosin regulatory light chain (MLC2). This integrated study uses site-directed mutagenesis to examine the relationship between removal of the myosin light chain kinase (MLCK) phosphorylation site, in vivo function of the flight system (flight tests, wing kinematics, metabolism, power output), isolated IFM fiber mechanics, MLC2 isoform pattern, and sarcomeric ultrastructure. The MLC2 mutants exhibit graded impairment of flight ability that correlates with a reduction in both IFM and flight system power output and a reduction in the constitutive level of MLC2 phosphorylation. The MLC2 mutants have wild-type IFM sarcomere and cross-bridge structures, ruling out obvious changes in the ultrastructure as the cause of the reduced performance. We describe a viscoelastic model of cross-bridge dynamics based on sinusoidal length perturbation analysis (Nyquist plots) of skinned IFM fibers. The sinusoidal analysis suggests the high power output of Drosophila IFM required for flight results from a phosphorylation-dependent recruitment of power-generating cross-bridges rather than a change in kinetics of the power generating step. The reduction in cross-bridge number appears to affect the way mutant flies generate flight forces of sufficient magnitude to keep them airborne. In two MLC2 mutant strains that exhibit a reduced IFM power output, flies appear to compensate by lowering wingbeat frequency and by elevating wingstroke amplitude (and presumably muscle strain). This behavioral alteration is not seen in another mutant strain in which the power output and estimated number of recruited cross-bridges is similar to that of wild type.     

Functional aspects of skeletal muscle contractile apparatus and sarcoplasmic reticulum after fatigue

This study examined the effects of fatigue on the functionalaspects of the contractile apparatus and sarcoplasmic reticulum (SR).Frog semitendinosus muscles were stimulated to fatigue, and skinnedfibers or a homogenate fraction was prepared from both fatigued andrested contralateral muscles. In fatigued fibers, maximalCa²⁺-activated force of thecontractile apparatus was unaltered, whereas maximal actomyosin-ATPaseactivity was depressed by 20%. TheCa²⁺ sensitivity of force wasincreased, whereas that of actomyosin-ATPase was not altered. Also, therate constant for tension redevelopment was decreased at submaximalCa²⁺ concentration. These latterfindings suggest that fatigue slows the dissociation offorce-generating myosin cross bridges.Ca²⁺ uptake andCa²⁺-ATPase activity of the SRwere depressed by 46 and 21%, respectively, in the fatigued muscles.Fatigue also reduced the rates of SR Ca²⁺ release evoked byAgNO₃ and4-chloro-m-cresol by 38 and 45%, respectively. During fatigue, the contractile apparatus and SR undergointrinsic functional alterations. These changes likely result inaltered force production and energy consumption by the intact muscle.

     

Changes of action potentials and force at lowered [Na+]o in mouse skeletal muscle: implications for fatigue

We examined 1) whether the effects of lowered trans-sarcolemmal Na⁺ gradient on force differed between nonfatigued fast- and slow-twitch muscles of mice and 2) whether effects on action potentials could explain the decrease of force. The Na⁺ gradient was reduced by lowering the extracellular [Na⁺] ([Na⁺]_o). The peak force-[Na⁺]_o relationships for the twitch and tetanus were the same in nonfatigued extensor digitorum longus and soleus muscles: force was maintained over a large range of [Na⁺]_o and then decreased abruptly over a much smaller range. However, fatigue was significantly exacerbated at a lowered [Na⁺]_o that had little effect in nonfatigued soleus muscle. This finding suggests that substantial differences exist in the Na⁺ effect on force between nonfatigued and fatigued muscle. The reduced contractility in nonfatigued muscles at lowered [Na⁺]_o was largely due to 1) an increased number of inexcitable fibers and threshold for action potentials, 2) a reduction of action potential amplitude, and 3) a reduced capacity to generate action potentials throughout trains. sodium gradient; muscle contraction; action potential train; extensor digitorum longus; soleus     

Mechanical strain memory in airway smooth muscle

We investigated the effect of a singlerapid stretch on poststretch force and myosin phosphorylation in bovinetracheal smooth muscle. When unstimulated muscle strips were stretchedfrom suboptimal length to optimal length (L_o),poststretch steady-state force was not significantly different fromthat of unstretched control at L_o. However, whencarbachol-activated muscle strips were stretched from suboptimal lengthto L_o, poststretch force and myosin phosphorylation were lower than control and significantly correlated with initial length. When poststretch muscle strips were allowed to relax for 1 hand then activated by K⁺ depolarization, the developedforce remained significantly correlated with initial length. When thesame strain was applied in 23 increments to minimize peak stress,poststretch force and myosin phosphorylation increased significantly,approaching the levels expected at L_o. Furthermore,poststretch force development increased after each cycle of contractionand relaxation, approaching the control level after four cycles. Theseresults suggest that activated airway smooth muscle cells can retainrelatively precise memory of past strain when they are stretchedrapidly with high stress.

     

Denervation produces different single fiber phenotypes in fast- and slow-twitch hindlimb muscles of the rat

Using a single, mechanically skinned fiber approach, we tested the hypothesis that denervation (0 to 50 days) of skeletal muscles that do not overlap in fiber type composition [extensor digitorum longus (EDL) and soleus (SOL) muscles of Long-Evans hooded rats] leads to development of different fiber phenotypes. Denervation (50 day) was accompanied by 1) a marked increase in the proportion of hybrid IIB/D fibers (EDL) and I/IIA fibers (SOL) from 30% to >75% in both muscles, and a corresponding decrease in the proportion of pure fibers expressing only one myosin heavy chain (MHC) isoform; 2) complex muscle- and fiber-type specific changes in sarcoplasmic reticulum Ca²⁺-loading level at physiological pCa 7.1, with EDL fibers displaying more consistent changes than SOL fibers; 3) decrease by 50% in specific force of all fiber types; 4) decrease in sensitivity to Ca²⁺, particularly for SOL fibers (by 40%); 5) decrease in the maximum steepness of the force-pCa curves, particularly for the hybrid I/IIA SOL fibers (by 35%); and 6) increased occurrence of biphasic behavior with respect to Sr²⁺ activation in SOL fibers, indicating the presence of both slow and fast troponin C isoforms. No fiber types common to the two muscles were detected at any time points (day 7, 21, and 50) after denervation. The results provide strong evidence that not only neural factors, but also the intrinsic properties of a muscle fiber, influence the structural and functional properties of a particular muscle cell and explain important functional changes induced by denervation at both whole muscle and single cell levels. mechanically skinned fibers; myosin heavy chain isoforms; lineage; sarcoplasmic reticulum; Ca²⁺; Sr²⁺ sensitivity; Long-Evans hooded rat     

Pliometric contraction-induced injury of mouse skeletal muscle: effect of initial length

Hunter, Kam D., and John A. Faulkner. Pliometriccontraction-induced injury of mouse skeletal muscle: effect of initial length. J. Appl. Physiol. 82(1):278-283, 1997.For single pliometric (lengthening) contractionsinitiated from optimal fiber length (L_f), the mostimportant factor determining the subsequent force deficit is the workinput during the stretch. We tested the hypothesis that regardless ofthe initial length, the force deficit is primarily a function of thework input. Extensor digitorum longus muscles of mice were maximallyactivated in situ and lengthened at 2 L_f /s from oneof three initial fiber lengths (90, 100, or 120% of L_f) to one ofthree final fiber lengths (150, 160, or 170% of L_f). Maximalisometric force production was assessed before and after the pliometriccontraction. No single mechanical factor, including thework input(r² = 0.34), was sufficient to explain the differences in force deficits observed among groups. Therefore, the force deficit appears to arisefrom a complex interaction of mechanicalevents. With the data grouped by initial fiber length,the correlation between the average work and the average force deficitwas high(r² = 0.97-0.99). Consequently, differences in force deficits among groups were best explained on the basis of the initial fiber length andthe work input during the stretch.

     

Five Alternative Myosin Converter Domains Influence Muscle Power,Stretch Activation,and Kinetics

Muscles have evolved to power a wide variety of movements. A protein component critical to varying power generation is the myosin isoform present in the muscle. However, how functional variation in muscle arises from myosin structure is not well understood. We studied the influence of the converter, a myosin structural region at the junction of the lever arm and catalytic domain, using Drosophila because its single myosin heavy chain gene expresses five alternative converter versions (11a–e). We created five transgenic fly lines, each forced to express one of the converter versions in their indirect flight muscle (IFM) fibers. Electron microscopy showed that the converter exchanges did not alter muscle ultrastructure. The four lines expressing converter versions (11b–e) other than the native IFM 11a converter displayed decreased flight ability. IFM fibers expressing converters normally found in the adult stage muscles generated up to 2.8-fold more power and displayed up to 2.2-fold faster muscle kinetics than fibers with converters found in the embryonic and larval stage muscles. Small changes to stretch-activated force generation only played a minor role in altering power output of IFM. Muscle apparent rate constants, derived from sinusoidal analysis of the chimeric converter fibers, showed a strong positive correlation between optimal muscle oscillation frequency and myosin attachment kinetics to actin, and an inverse correlation with detachment related cross-bridge kinetics. This suggests the myosin converter alters at least two rate constants of the cross-bridge cycle with changes to attachment and power stroke related kinetics having the most influence on setting muscle oscillatory power kinetics.     

The roles of myosin ATPase activity and myosin light chain relative content in the slowing of type IIB fibers with hindlimb unweighting in rats

We tested the hypothesis that slowing of shortening velocity generated by type IIB fibers from hindlimb-unweighted (HU) rats resulted from a reduced ATPase activity and/or a reduction in the relative content of myosin light chain 3f isoform content (MLC_3f). After 2, 3, and 4 wk of HU, maximal unloaded shortening velocity (V_o) of single permeabilized semimembranosus muscle fibers was determined by the slack test. Subsequently, the myosin heavy chain and the relative content of MLC were determined by SDS-PAGE. The ratio of MLC_3f to MLC_2f was determined by densitometric analysis. In addition, myofibrils were prepared from permeabilized fibers (soleus and semimembranosus muscles) and assayed for resting myosin ATPase and Ca²⁺-activated myosin ATPase. After HU, V_o declined by 28–40% and the MLC_3f/MLC_2f ratio decreased by 32 to 48%. A significant correlation between the relative amount of MLC_3f and V_o was found (r = 0.48, P < 0.05). Resting myosin ATPase rates were not different between myofibrils prepared from corresponding muscles of control and HU rats (P = 0.86). Ca²⁺-activated myosin ATPase activities also were not different between myofibrils prepared from corresponding muscles of control and HU rats (P = 0.13). These data suggest that the slowing of maximal unloaded shortening velocity in type IIB fibers with HU is, at least in part, due to a relative change in the essential light chain composition, a decrease in the relative amount of MLC_3f and most likely a concomitant increase in MLC_1f. However, this reduction in V_o is independent of myosin ATPase activity. unloading shortening velocity; myosin light chain 3f     

An Embryonic Myosin Isoform Enables Stretch Activation and Cyclical Power in Drosophila Jump Muscle

The mechanism behind stretch activation (SA), a mechanical property that increases muscle force and oscillatory power generation, is not known. We used Drosophila transgenic techniques and our new muscle preparation, the jump muscle, to determine if myosin heavy chain isoforms influence the magnitude and rate of SA force generation. We found that Drosophila jump muscles show very low SA force and cannot produce positive power under oscillatory conditions at pCa 5.0. However, we transformed the jump muscle to be moderately stretch-activatable by replacing its myosin isoform with an embryonic isoform (EMB). Expressing EMB, jump muscle SA force increased by 163% and it generated net positive power. The rate of SA force development decreased by 58% with EMB expression. Power generation is Pi dependent as >4 mM Pi was required for positive power from EMB. Pi increased EMB SA force, but not wild-type SA force. Our data suggest that when muscle expressing EMB is stretched, EMB is more easily driven backward to a weakly bound state than wild-type jump muscle. This increases the number of myosin heads available to rapidly bind to actin and contribute to SA force generation. We conclude that myosin heavy chain isoforms influence both SA kinetics and SA force, which can determine if a muscle is capable of generating oscillatory power at a fixed calcium concentration.     

The Metabolism of cis ABA-aldehyde by the Wilty Mutants of Potato, Pea and Arabidopsis thaliana

RS-[²H₁] cis ABA-aldehyde was fed to ABA-deficient mutants ofpotato (droopy), pea (wilty) and Arabidopsis thaliana (aba¹)along with appropriate non-mutant controls. Both the wilty andaba¹ mutants readily oxidized the monodeuterated ABA-aldehydeto ABA. The incorporation of label into ABA by these two mutantswas indistinguishable from that detected in the non-mutant controls.In contrast, the droopy mutants poorly incorporated the labelledprecursor into ABA. Instead they reduced and isomerized RS-[²H₁] cis ABA-aldehyde to a mixture of 2, cis and 2, trans ABA-alcohols.Thus the droopy mutant affects the last step in ABA biosynthesis,a position it shares with the tomato mutants, flacca and sitiens.Genetic evidence suggesting that droopy and sitiens may be correspondinggene loci is discussed. Key words: ABA metabolism, wilty mutants, pea, potato, Arabidopsis     

Photoinhibition and light-induced cyclic electron transport in ndhB(-) and psaE(-) mutants of Synechocystis sp. PCC 6803

The ndhB^– and psaE^– mutants of the cyanobacteriumSynechocystis sp. PCC 6803 are partly deficient in PSI-drivencyclic electron transport. We compared photoinhibition in thesemutants to the wild type to test the hypothesis that PSI cyclicelectron transport protects against photoinhibition. Photoinhibitorytreatment greatly accelerated PSI cyclic electron transportin the wild type and also in both the mutants. The psaE^–mutant showed rates of PSI cyclic electron transport similarto the wild type under all conditions tested. The ndhB^–mutant showed much lower rates of PSI cyclic electron transportthan the wild type following brief dark adaptation but exceededwild type rates after exposure to photoinhibitory light. Thewild type and both mutants showed similar rates of photoinhibitiondamage and photoinhibition repair at PSII. Photoinhibition atPSI was much slower than at PSII and was also similar betweenthe wild type and both mutants, despite the known instabilityof PSI in the psaE^– mutant. We conclude that photoinhibitorylight induces sufficient PSI-driven cyclic electron transportin both the ndhB^– and psaE^– mutants to fulfill anyrole that cyclic electron transport plays in protection againstphotoinhibition. ⁴ Corresponding author: E-mail, sherbert@uwyo.edu; Fax, +1-307-766-2851;Phone, +1-307-766-4353.