In this article we will discuss about the regulation of zein gene.
The molecular biology of the zein gene system is well established in plant system. Comparison of genomic and cDNA clones have shown that they are totally devoid of intervening sequences. The regulatory signals of zein is TATA box, which is found 30 bp upstream of the transcriptional initiation site and in one case, active second promoter is present 900 bp from the regulatory site of transcription initiation.
Although the presences of CAAT box have been identified but different consensus sequence can be found around the site 80 when zein genes and number of other genes are compared. Its regulatory function is not known, however, it might acts as enhancer sequence as it bears certain core enhancer sequence GTGGAAA.
Zein genes are clustered and have TTT in some interesting implications for the evolution and conservation of their structure. The close linkage groups of zein genes based genetic mapping list out certain peculiarities. The genes of different subfamilies are quite diverged and genes are quite old in evolutionary terms. Zein genes have a high proportion of codes (32%). They can mutant into a sharp codon in a single step.
For example, high level of glutamine in zein gene can trigger mutant process. It is because glutamine is encoded by CAA and CAG and C to U transition occurs frequently. Therefore, frequency of stop codon production in zein gene is twice (6.8%) as that of mutation of average gene. The active functioning of the genes highlights the structure, presence and location of signal sequences.
The signal sequences sometimes repeat to several times. For example, it is repeated three times in 5′ non translated regions of all zein messengers and this repeat explains the efficiency of translated zein messenger. Generally, initiator signal or mRNA is to be recognized by ribosomes. Need for the translation of different mRNA with different efficiency is due to different signal difference.
The initiator codon for the translation of zein is (all cases) AUG triplet on the mRNA and 20 to 21 codon upstream from the start of the codon sequences for zein. The consensus sequences surrounds the initiator AUG in plant gene is 5′ NNNNANA/UNU/AANNNNANNAUGGCU3′. Overall more variation of initiation signal can be observed for plant genes (Table 3.1).
Most plant genes have more than one of the consensuses for the polyadenylation signal. Polyadenylation signal of plant differs from animal conserved sequences frequently. Stability of mRNA for histone protein has been examined. Histone mRNA stability is regulated at cell cycle itself. It is particularly at S phase of cell cycle.
Synthesis of high amount of DNA during S phase demands considerable amount of histone for association into nucleoprotein complex. Degradation mRNA takes place immediately once the phase of cell cycle is completed. Regulations of histone mRNA stability are due to short 3′ stop-loop structure and replace poly (A) tail found in other mRNA.