Plastome mutants

A chloroplast location of the gene for Cyt / was suggested by two plastome mutants of Oenothera which were deficient in spectrally detected Cyt/[102], and by the maternal mode of inheritance of Cyt /in interspecific Fj hybrids of Nicotiana [103]. The structural gene (petA) for the Cyt / polypeptide was initially localized and characterized from pea chloroplast DNA [104,105], and subsequently from wheat [106], spinach [107] and Oenothera [108]. Open reading frames of 320 codons have been detected in chloroplast DNA from each of these plants. Comparison with the determined N-terminal sequence of Cyt / indicates the presence of a 35 amino acid residue pre-sequence and a mature polypeptide of 285 amino acid residues. A putative haem-binding site, Cys-Ala-Asn-Cys-His, is located near the... 

It is relatively rare to find significant quantities of both odd chain length and branched chain fatty acids in biological systems. However such compounds have been Isolated and identified from the epicuticular wax from Brussels sprout leaves, with anteiso-C j and anteiso-C making up 36% of the total saturated fatty acid fraction (13). Similarly Radunz has isolated and identified iso and anteiso mono methyl branched fatty acids in phospholipids, contained in yellow-white leaves and petals of the plastome mutants "Prasinizans" of Antirrhinum majus and "Xanthr of Nicotiana tabacum M4T In addition small amounts of odd chain length fatty acids and alcohols have been found in the surface waxes of Zea mays husks (15). 

Frame shift mutation In Chlamydomonas reinhardtii the plastome mutant F U026 proved to be a frame shift mutation in the psa3 gene, caused by a 4 base pair deletion. This frame shift leads to a truncated CPI apoprotein, because polypeptide synthesis is terminated 7 codons (amino acids) behind the deletion site (23). 

A plastome mutant with a deficient photoevetem II has been deecribed in Antirrhinum majus (mutant viridis-1 /33/). The deficiency in photoeyetem II is correlated with a lose and a reduction respectively of thylakoid bands associated with photoeyetem II. Moreover, it should be kept in mind that all the different atrazine resistant mutants (listed in Table 1) have alterations in the protein Dl (= Qg), which is a constitutive part of the photosystem II complex. Most of them have a reduced photosynthetic capacity. 

On the other hand, a plastome mutant (thm-u-i) has been found in Chlamydomonas reinhardtli, which has a variant thylakoid protein (polypeptide 5 is about 1000 dalton larger than its wild type counterpart 5) and nevertheless a normal photosynthetic capacity (34). Mutations in the plaetid gene rbcL, coding for the LS of rubisco, lead to mlseenee and nonsense codons, as referred to in the preceeding paragraphs. They result in the synthesis of unstable or truncated polypeptides, which arte nonfunctional and thus block the dark reactions of photosynthesis. 

A substantial difference between chloroplasts of different photosynthetic activities appears to exist with respect to the phospholipid content. Whereas in normally developed chloroplasts of green leaf areas the phospholipid content is approximately 7%, this content increases in chloroplasts with exclusively photosystem I activity to 14% and increases further in chloroplasts with increased photosystem I activity to 28% of the total lipid content whereas it appears that the ratio of phosphatidylglycerol to phosphatidylcholine and tp phosphatidylinositol is the same in the two types of chloroplasts of the plastome mutant. Chloroplasts of the tobacco mutant NC 95 with only photosystem I activity seem to contain besides phosphat i dyl i nosi tol essentiall-y only phosphatidylcholine as the main component. 

Table 1 Comparison of the lipid composition of chloroplasts and. leaves of the plastome mutant... 

Table 2 Fatty acids composition of leaf and chloroplast lipids of the plastome mutant of...

Mono-methyl branched fatty acids of the iso- and anteiso types with chain lengths of 12 to 22 carbon atoms were found in yellow-white leaves and petals of the plastome mutants "Prasinizans" of Antirrhinum majus and "Xanthi"ot Nicotiana tabacum. These branched chain fatty acids are preponderantly esterified in the sn-2 position of the glycerol backbone of the membrane phospholipids namely phosphatidylcholine, phosphatidyletha-nolamine and phosphatidylinositol. The phospholipids appear to contain 16-24 per cent iso- and anteiso fatty acids, 19-47 per cent unsaturated and 38-63 per cent saturated fatty acids. The methyl-branched chain fatty acids seem to take over the function of the unsaturated fatty acids. 

Iso- and anteiso fatty acids, phosphatidylcholine, phosphatidyletha-nolamine, phosphatidylinositol, cell membrane, plastome mutants of Antirrhinum majus "Prasinizans" and of Nicotiana tabacum "Xanthi". 

In a previous publication we had reported that yellow-white leaves of the plastome mutant "Prasinizans" of Antirrhinum majus contained methyl-branched fatty acids (1). They make up for 6-8 per cent of the total fatty acids. The methyl-branched fatty acids were isolated via urea adducts and characterized by means of comparative gas chromatography with authentic fatty acids (2,3) by IR-spectroscopy, NMR-spectroscopy and by means of the measured optical activity as mono- and methyl-branched fatty acids of the iso- and anteiso-types (1). 

Recent investigations have shown that these iso- and anteiso acids also occur in yellow-white leaves of the plastome mutant "Xanthi" of Nicotiana tabacum. The fatty acids were detected in the phosphatides of both plants and make up for 16-24 per cent of the fatty acids of the phospholipids. Phospholipids are known to be functional components of the lipid matrix of the cell membrane as well as of membranes of various cell organelles which allow biological activity in cooperation with other membrane components. ... 

Table Hi Positional Distribution of Fatty Acids between the sn-1 and sn-2 Position at the Glycerol Backbone in Phospholipids of Petals and Yellow-White Leaves of the Plastome Mutant "Prasinizans" of Antirrhinum majus.

---