Translational repression: biological activity of plasmid-encoded bacteriophage T4 RegA protein

The RegA protein of bacteriophage T4 is a translational repressor that regulates expression of several phage early mRNAs. We have cloned wild-type and mutant alleles of the T4 regA gene under control of the heat-inducible, plasmid-borne leftward promoter (PL) of phage lambda. Expression of the cloned regA+ gene resulted in the synthesis of a protein that closely resembled phage-encoded RegA protein in biological properties. It repressed its own synthesis (autogenous translational control) as well as the synthesis of specific T4-encoded proteins that are known from other studies to be under RegA-mediated translational control. Cloned mutant alleles of regA exhibited derepressed synthesis of the mutant regA gene products and were ineffective in trans against RegA-sensitive mRNA targets. The effects of plasmid-encoded RegA proteins were also demonstrated in experiments using two compatible plasmids in uninfected Escherichia coli. The two-plasmid assays confirm the sensitivities of several cloned T4 genes to RegA-mediated translational repression and are well-suited for genetic analysis of RegA target sites. Repression specificity in this system was demonstrated by using wild-type and operator-constitutive translational initiation sites of T4 rIIB fused to lacZ. The results show that no additional T4 products are required for RegA-mediated translational repression. Additional evidence is provided for the proposal that uridine-rich mRNA sequences are preferred targets for the repressor. Surprisingly, plasmid-generated RegA protein represses the synthesis of some E. coli proteins and appears to enhance selectively the synthesis of others. The RegA protein may have multiple functions, and its binding sites are not restricted to phage mRNAs.     

Motile aeromonas infection of striped (grey) mullet Mugil cephalus

Infection of striped mullet (Mugil cephalus) with Aeromonas hydrophila results in an acute septicemic disease. The disease can be experimentally induced by intramuscular injection, skin or gill scarification or by the oral route using pellets purposely seeded with bacteria. The organism was isolated from the blood 1–2 days after infection and from all organs 24 hr or longer after infection. The disease is characterized by early inflammatory and proliferative changes and later necrotic changes. Enteritis and hepatic necrosis are constant findings in aeromonad disease of M. cephalus but surface lesions are not pathognomic for these infections in mullet. Death of infected fish may be attributed to bacterial toxins which cause necrosis of parenchymal organs and soft tissue structure.     

Studies on the effect of NAD(H) on nitrogenase activity in Rhodospirillum rubrum

The effect of NAD(P) and analogs of this nucleotide on nitrogenase activity in Rhodospirillum rubrum has been studied. Addition of NAD⁺ to nitrogen fixing Rsp. rubrum leads to inhibition of nitrogenase. NADP⁺ has the same effect but NADH or analogs modified in the nicotinamide portion do not cause inhibition. In contrast to ammonium ions, addition of NAD⁺ leads to inhibition of nitrogenase in cells that have been N-starved under argon. The inhibitory effect of NAD⁺ is more pronounced at lower light intensities. Addition of NAD⁺ also leads to inhibition of glutamine synthetase, a phenomenon also occurring when “switchoff” is produced by the addition of effectors such as ammonium ions or glutamine. It is also shown that NAD⁺ is taken up by Rsp. rubrum cells.     

Chromosome association and pollen fertility of parental and interspecific hybrids of Lycopersicon esculentum X L. hirsutum and L. pennellii

Cytological studies were carried out on two wild species (L. hirsutum and L. pennellii) and the cultivated species (L. esculentum) of tomato and their F1 hybrids. Both parents and hybrids show a diploid chromosome number of 2n=24. The meiotic behaviour of the cultivated species showed a high degree of chromosome homology resulting in a high level of chiasmata frequency per bivalent. In contrast, the two wild species showed a slight increase in uniyalent frequency and a decrease in bivalent formation and chiasmata frequency. The meiotic behaviour of the hybrids showed a high level of univalents and low levels of bivalents as well as trivalents. Highly significant decreases in chiasmata frequency and increases in meiotic abnormalities, especially in the L. esculentum X L. pennellii hybrid, also were detected. The high meiotic irregularity and low chiasmata frequency recorded in the second hybrid indicated the disharmony and difference between its parental genomes and also served to predict its sterility. With regard to degree of pairing recorded in the hybrids, there is a possibility that sterility in such cases may refer to genetic factors in addition to the previously mentioned reasons. Pollen fertility showed no great difference between L. esculentum and L. hirsutum and their F1 hybrid, but a significant decrease was recorded in the L. esculentum X L. pennellii hybrid, which was clearly associated with high meiotic irregularity, low chiasmata frequency and chromosome association.