On gene symbolization for the second sucrose synthase enzyme

Two sucrose synthase (SS) isozymes in maize have been known for the past several years (Chourey and Nelson, Biochem. Genet. 14:1041, 1976, and Genetics 91:317, 1979). The Sh-locus-encoded enzyme is unique with respect to its abundant levels in the Sh endosperm.

The genetic basis for the second sucrose synthase isozyme, which was first uncovered through the analysis of sh mutants, was however not firmly established initially. Although a hypothesis that the second enzyme is due to a second gene (i.e., non-allelic to Sh) was favoured, an alternative idea that it was due to leaky expression of the sh allele in sh genotype could not be ruled out unequivocally. In this regard, the SS analyses of the sh-deletion strain (sh bz-m4) were quite useful, since the deletion stock proved to be similar to several spontaneous sh mutants, providing the first conclusive evidence that a second SS gene was present in the maize genome (Chourey, MGG 184:372, 1981). In addition, it was also possible to demonstrate reproducibly that there were two SS proteins in Sh endosperm, thus demonstrating that both genes are expressed in homozygous Sh endosperm (MGG 184:372, 1981). McCormick et al. (MGG 187:494, 1982) showed molecular similarity between the two SS encoding genes. Although no formal attempt was made to name the second gene, the most obvious and practical symbol, Sucrose synthase 2 (Ss2) for the gene and SS2 for its enzyme, have been used in the past several publications originating from this laboratory on the implicit assumption that the Sh encodes the SS1 enzyme. During the 1981-1986 period, at least 4-5 presentations on various aspects of the Ss2 gene have also been made at the Annual Maize Genetics meetings. Interestingly, no concerns or comments to express change in the Ss2 symbol were raised at these meetings, the best possible forum for the discussion of such matters. A recent publication, however (McCarty et al., PNAS 83:9099, 1986), uses an entirely new name, constitutive sucrose synthase (Css), instead of the previous designation, Ss2. A casual rejection of the Ss2 symbol is surprisingly uncorroborated by any statements to justify the action. Ironically, McCarty et al. used the Ss2 symbol to discuss the present data at the 29th Maize Genetics meetings (1986). The main intent of this communication is to point out that the Css symbol for the Ss2 gene is inappropriate for the following reasons:

(1) Ss2 expression, in contrast to McCarty et al.'s claim, is not constitutive in maize plants. Although the statement is made that "Css protein has been found in all maize tissues so far studied", (my underlining), an assertion of such critical importance is surprisingly unsubstantiated either by data or by reference to previous publications. This information is critical, since a previous extensive study of starch synthesis enzymes in endosperm and pollen has detected no SS activity in mature or immature pollen (Bryce and Nelson, Plant Phys. 63:312, 1979). I have similarly not detected any SS activity in pollen derived from plants of Sh or sh genotype (unpubl. data). The Css symbol would thus need to specify the tissues where the gene is expressed constitutively.

(2) In principle, gene symbolization on the basis of regulatory properties is unprecedented in maize. Usually, one needs a name for the gene before it is analyzed. This may explain why some of the genes, particularly those which are better characterized for their regulatory properties (e.g., Adh1 and Adh2) than the SS encoding genes, are symbolized on a neutral basis such as the order of their discovery. Furthermore, I believe, we know very little about the regulatory properties of Sh and Ss2 at the present time and it is extremely premature to label one as Css. The successful use of Western blot analysis on denaturing gels to examine SS specific proteins and the recovery of gene specific probes to analyze transcripts will, however, soon fulfill this deficiency. Neutral symbols such as Sh and Ss2 allow such analyses to be done independently of their regulatory properties.

(3) Traditionally, constitutive refers to some constant level of expression at the protein/enzyme level, although it could include expression at any point in the path of gene expression. The fact that one can now incisively analyze various steps in gene expression and define points of blockage, demands that greater care be taken in gene symbolization. It is entirely possible that Ss2 could be constitutive at the transcription level but highly regulated at the protein level (an on-going project in my lab). Indeed, SS2 protein as well as the enzyme levels in leaf tissue of one-week and nearly four-week-old seedlings of the sh genotype vary by more than an order of magnitude (unpubl. data); transcription analyses are presently in progress. The point is that the designation "constitutive" in gene symbolization, as in the case of the Css symbol, sets an undesirable precedent and appears ambiguous due to the lack of information regarding the level of analysis or specifically, the level at which the gene is constitutively expressed.

In summary, although gene symbolization for SS enzymes is a bit problematic due to certain unusual constraints (e.g., the Sh symbol cannot be changed to associate with the first of the two sucrose synthases - the SSU the symbol Css makes matters worse. The most important consideration, however, is that Ss2 expression in the maize plant is not constitutive, and thus the usage of Css is technically incorrect and highly misleading.

Prem Chourey

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